Fix bint on x64, and make bextend consistent with bool representation.

There has been occasional confusion with the representation that we use
for bool-typed values in registers, at least when these are wider than
one bit. Does a `b8` store `true` as 1, or as all-ones (`0xff`)?

We've settled on the latter because of some use-cases where the wide
bool becomes a mask -- see #2058 for more on this.

This is fine, and transparent, to most operations within CLIF, because
the bool-typed value still has only two semantically-visible states,
namely `true` and `false`.

However, we have to be careful with bool-to-int conversions. `bint` on
aarch64 correctly masked the all-ones value down to 0 or 1, as required
by the instruction specification, but on x64 it did not. This PR fixes
that bug and makes x64 consistent with aarch64.

While staring at this code I realized that `bextend` was also not
consistent with the all-ones invariant: it should do a sign-extend, not
a zero-extend as it previously did. This is also rectified and tested.
(Aarch64 also already had this case implemented correctly.)

Fixes #3003.

cranelift/filetests/filetests/isa/x64/bextend.clif

@@ -0,0 +1,16 @@
+test compile
+target x86_64 machinst
+
+function %f0(b8) -> b64 {
+block0(v0: b8):
+  v1 = bextend.b64 v0
+  return v1
+}
+
+; check:  pushq   %rbp
+; nextln: movq    %rsp, %rbp
+; nextln: movsbq  %dil, %rsi
+; nextln: movq    %rsi, %rax
+; nextln: movq    %rbp, %rsp
+; nextln: popq    %rbp
+; nextln: ret

cranelift/filetests/filetests/isa/x64/cmp-mem-bug.clif

@@ -11,7 +11,7 @@ block0(v0: i64, v1: i64):
     v4 = bint.i64 v3
 ; nextln: cmpq    %rax, %rdi
 ; nextln: setz    %cl
-; nextln: movzbq  %cl, %rcx
+; nextln: andq    $$1, %rcx
 
     v5 = select.i64 v3, v0, v1
 ; nextln: cmpq    %rax, %rdi
@@ -34,7 +34,7 @@ block0(v0: f64, v1: i64):
 ; nextln: setnp   %dil
 ; nextln: setz    %sil
 ; nextln: andl    %edi, %esi
-; nextln: movzbq  %sil, %rsi
+; nextln: andq    $$1, %rsi
 
     v5 = select.f64 v3, v0, v0
 ; nextln: ucomisd %xmm1, %xmm0

cranelift/filetests/filetests/isa/x64/i128.clif

@@ -463,7 +463,8 @@ block0(v0: b1):
     v1 = bint.i128 v0
     return v1
 
-; check:  movzbq  %dil, %rsi
+; nextln: movq    %rdi, %rsi
+; nextln: andq    $$1, %rsi
 ; nextln: xorq    %rdi, %rdi
 ; nextln: movq    %rsi, %rax
 ; nextln: movq    %rdi, %rdx