[codegen] add encodings for isub borrow variants

Add encodings for isub borrow variants (isub_bout, isub_bin,
isub_borrow) for x86_32, enabling the legalization for isub.i64 to work.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1576675
Bug: https://github.com/CraneStation/cranelift/issues/765

cranelift/filetests/filetests/isa/x86/binary32.clif

@@ -477,6 +477,14 @@ ebb0:
     ; asm: adcl %esi, %ecx
     [-,%rcx,%rflags] v704, v705 = iadd_carry v1, v2, v702 ; bin: 11 f1
 
+    ; Borrow Subtraction
+    ; asm: subl %esi, %ecx
+    [-,%rcx,%rflags] v706, v707 = isub_bout v1, v2         ; bin: 29 f1
+    ; asm: sbbl %esi, %ecx
+    [-,%rcx] v708 = isub_bin v1, v2, v707                  ; bin: 19 f1
+    ; asm: sbbl %esi, %ecx
+    [-,%rcx,%rflags] v709, v710 = isub_borrow v1, v2, v707 ; bin: 19 f1
+
     ; asm: testl %ecx, %ecx
     ; asm: je ebb1
     brz v1, ebb1                                ; bin: 85 c9 74 0e

cranelift/filetests/filetests/isa/x86/legalize-i64.clif

@@ -8,9 +8,20 @@ function %iadd(i64, i64) -> i64 {
 ebb0(v1: i64, v2: i64):
     v10 = iadd v1, v2
     ; check: v1 = iconcat $(v1_lsb=$V), $(v1_msb=$V)
-    ; check: v2 = iconcat $(v2_lsb=$V), $(v2_msb=$V)
-    ; check: $(v10_lsb=$V), $(carry=$V) = iadd_cout $v1_lsb, $v2_lsb
-    ; check: $(v10_msb=$V) = iadd_cin $v1_msb, $v2_msb, $carry
-    ; check: v10 = iconcat $v10_lsb, $v10_msb
+    ; nextln: v2 = iconcat $(v2_lsb=$V), $(v2_msb=$V)
+    ; nextln: $(v10_lsb=$V), $(carry=$V) = iadd_cout $v1_lsb, $v2_lsb
+    ; nextln: $(v10_msb=$V) = iadd_cin $v1_msb, $v2_msb, $carry
+    ; nextln: v10 = iconcat $v10_lsb, $v10_msb
+    return v10
+}
+
+function %isub(i64, i64) -> i64 {
+ebb0(v1: i64, v2: i64):
+    v10 = isub v1, v2
+    ; check: v1 = iconcat $(v1_lsb=$V), $(v1_msb=$V)
+    ; nextln: v2 = iconcat $(v2_lsb=$V), $(v2_msb=$V)
+    ; nextln: $(v10_lsb=$V), $(borrow=$V) = isub_bout $v1_lsb, $v2_lsb
+    ; nextln: $(v10_msb=$V) = isub_bin $v1_msb, $v2_msb, $borrow
+    ; nextln: v10 = iconcat $v10_lsb, $v10_msb
     return v10
 }