x64: Add shuffle cases for punpck{h,l}bw (#5905)

* x64: Add `shuffle` cases for `punpck{h,l}bw`

I noticed this difference between LLVM and Cranelift for something I was
looking at recently, and while it's probably not all that common I
figured I'd add it here since it should be somewhat useful nevertheless.

* Review feedback

* Use u128 extractor instead

cranelift/filetests/filetests/isa/x64/shuffle.clif

@@ -0,0 +1,54 @@
+test compile precise-output
+set enable_simd
+target x86_64
+
+function %punpcklbw(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v2 = shuffle v0, v1, 0x17071606150514041303120211011000
+    return v2
+}
+
+; VCode:
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   punpcklbw %xmm0, %xmm1, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+; 
+; Disassembled:
+; block0: ; offset 0x0
+;   pushq %rbp
+;   movq %rsp, %rbp
+; block1: ; offset 0x4
+;   punpcklbw %xmm1, %xmm0
+;   movq %rbp, %rsp
+;   popq %rbp
+;   retq
+
+function %punpckhbw(i8x16, i8x16) -> i8x16 {
+block0(v0: i8x16, v1: i8x16):
+    v2 = shuffle v0, v1, 0x1f0f1e0e1d0d1c0c1b0b1a0a19091808
+    return v2
+}
+
+; VCode:
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   punpckhbw %xmm0, %xmm1, %xmm0
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+; 
+; Disassembled:
+; block0: ; offset 0x0
+;   pushq %rbp
+;   movq %rsp, %rbp
+; block1: ; offset 0x4
+;   punpckhbw %xmm1, %xmm0
+;   movq %rbp, %rsp
+;   popq %rbp
+;   retq
+