Move bitselect->vselect optimization to x64 back-end (#5191)

The simplifier was performing an optimization to replace bitselect
with vselect if the all bytes of the condition mask could be shown
to be all ones or all zeros.

This optimization only ever made any difference in codegen on the
x64 target.  Therefore, move this optimization to the x64 back-end
and perform it in ISLE instead.  Resulting codegen should be
unchanged, with slightly improved compile time.

This also eliminates a few endian-dependent bitcast operations.

cranelift/filetests/filetests/isa/x64/simd-bitselect.clif

@@ -0,0 +1,123 @@
+test compile precise-output
+set enable_simd
+target x86_64 skylake
+
+function %mask_from_icmp(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v2 = icmp eq v0, v1
+    v3 = bitselect v2, v0, v1
+    return v3
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movdqa  %xmm0, %xmm5
+;   pcmpeqb %xmm5, %xmm1, %xmm5
+;   movdqa  %xmm0, %xmm8
+;   movdqa  %xmm5, %xmm0
+;   movdqa  %xmm1, %xmm6
+;   pblendvb %xmm6, %xmm8, %xmm6
+;   movdqa  %xmm6, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %mask_from_fcmp(f32x4, f32x4, i32x4, i32x4) -> i32x4  {
+block0(v0: f32x4, v1: f32x4, v2: i32x4, v3: i32x4):
+    v4 = fcmp eq v0, v1
+    v5 = bitselect v4, v2, v3
+    return v5
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   cmpps   $0, %xmm0, %xmm1, %xmm0
+;   movdqa  %xmm3, %xmm8
+;   pblendvb %xmm8, %xmm2, %xmm8
+;   movdqa  %xmm8, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %mask_casted(i8x16, i8x16, i32x4) -> i8x16 {
+block0(v0: i8x16, v1: i8x16, v2: i32x4):
+    v3 = bitcast.i8x16 v2
+    v4 = bitselect v3, v0, v1
+    return v4
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movdqa  %xmm0, %xmm5
+;   pand    %xmm5, %xmm2, %xmm5
+;   movdqa  %xmm2, %xmm0
+;   pandn   %xmm0, %xmm1, %xmm0
+;   por     %xmm0, %xmm5, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %good_const_mask_i8x16(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v3 = vconst.i8x16 [0 0 0xFF 0 0 0xFF 0 0 0 0 0xFF 0 0 0 0 0xFF]
+    v4 = bitselect v3, v0, v1
+    return v4
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movdqa  %xmm0, %xmm6
+;   movdqu  const(0), %xmm0
+;   movdqa  %xmm6, %xmm8
+;   movdqa  %xmm1, %xmm6
+;   pblendvb %xmm6, %xmm8, %xmm6
+;   movdqa  %xmm6, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %good_const_mask_i16x8(i16x8, i16x8) -> i16x8 {
+block0(v0: i16x8, v1: i16x8):
+    v3 = vconst.i16x8 [0x0000 0xFF00 0x0000 0x00FF 0x0000 0xFFFF 0x00FF 0xFFFF]
+    v4 = bitselect v3, v0, v1
+    return v4
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movdqa  %xmm0, %xmm6
+;   movdqu  const(0), %xmm0
+;   movdqa  %xmm6, %xmm8
+;   movdqa  %xmm1, %xmm6
+;   pblendvb %xmm6, %xmm8, %xmm6
+;   movdqa  %xmm6, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+
+function %bad_const_mask(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v3 = vconst.i8x16 [0 0 0xF0 0 0 0xFF 0 0 0 0 0xFF 0 0 0 0 0xFF]
+    v4 = bitselect v3, v0, v1
+    return v4
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movdqu  const(0), %xmm6
+;   movdqa  %xmm6, %xmm9
+;   movdqa  %xmm0, %xmm5
+;   pand    %xmm5, %xmm9, %xmm5
+;   movdqa  %xmm9, %xmm0
+;   pandn   %xmm0, %xmm1, %xmm0
+;   por     %xmm0, %xmm5, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+

cranelift/filetests/filetests/isa/x64/simd-bitwise-compile.clif

@@ -128,28 +128,6 @@ block0(v0: i32x4, v1: i32x4):
 ;   popq    %rbp
 ;   ret
 
-function %bitselect_i16x8() -> i16x8 {
-block0:
-    v0 = vconst.i16x8 [0 0 0 0 0 0 0 0]
-    v1 = vconst.i16x8 [0 0 0 0 0 0 0 0]
-    v2 = vconst.i16x8 [0 0 0 0 0 0 0 0]
-    v3 = bitselect v0, v1, v2
-    return v3
-}
-
-;   pushq   %rbp
-;   movq    %rsp, %rbp
-; block0:
-;   movdqu  const(0), %xmm0
-;   movdqu  const(0), %xmm2
-;   movdqu  const(0), %xmm6
-;   pand    %xmm2, %xmm0, %xmm2
-;   pandn   %xmm0, %xmm6, %xmm0
-;   por     %xmm0, %xmm2, %xmm0
-;   movq    %rbp, %rsp
-;   popq    %rbp
-;   ret
-
 function %vselect_i16x8(i16x8, i16x8, i16x8) -> i16x8 {
 block0(v0: i16x8, v1: i16x8, v2: i16x8):
     v3 = vselect v0, v1, v2