Implement shift-immediate encodings for x86 (#283)

* add x86 encodings for shift-immediate instructions

implements encodings for ishl_imm, sshr_imm, and ushr_imm. uses 8-bit immediates.

added tests for the encodings to intel/binary64.cton. Canonical versions
come from llvm-mc.

* translate test to use shift-immediates

* shift immediate encodings: use enc_i32_i64

and note why the regular shift encodings cant use it above

* add additional encoding tests for shift immediates

this covers 32 bit mode, and 64 bit operations in 64 bit mode.

cranelift/filetests/isa/intel/legalize-div.cton

@@ -32,7 +32,7 @@ function %sdiv(i64, i64) -> i64 {
 ebb0(v0: i64, v1: i64):
     ; check: ebb0(
     v2 = sdiv v0, v1
-    ; check: $(hi=$V) = sshr
+    ; check: $(hi=$V) = sshr_imm
     ; nextln: $(d=$V), $(r=$V) = x86_sdivmodx v0, $hi, v1
     return v2
     ; nextln: return $d
@@ -46,7 +46,7 @@ ebb0(v0: i64, v1: i64):
     v2 = srem v0, v1
     ; nextln: $(fm1=$V) = ifcmp_imm v1, -1
     ; nextln: brif eq $fm1, $(m1=$EBB)
-    ; check: $(hi=$V) = sshr
+    ; check: $(hi=$V) = sshr_imm
     ; nextln: $(d=$V), $(r=$V) = x86_sdivmodx v0, $hi, v1
     ; nextln: jump $(done=$EBB)($r)
     ; check: $m1: