Fix x86 encoding of uextend/sextend from 8-bit inputs.

The x86-32 and non-REX encodings of movsbl and movzbl require one of
the ABCD registers as input.

cranelift/filetests/isa/x86/abcd.cton

@@ -0,0 +1,13 @@
+test regalloc
+isa x86
+
+; %rdi can't be used in a movsbl instruction, so test that the register
+; allocator can move it to a register that can be.
+
+function %test(i32 [%rdi]) -> i32 system_v {
+ebb0(v0: i32 [%rdi]):
+    v1 = ireduce.i8 v0
+    v2 = sextend.i32 v1
+    return v2
+}
+; check: regmove v1, %rdi -> %rax