Fix a type error in the legalizer patterns.

The carry and borrow values are boolean, so we have to convert them to
an integer type with bint(c) before we can add them to the result.

Also tweak the default legalizer action for unsupported types: Only
attempt a narrowing pattern for lane types > 32 bits.

This was found by @angusholder's new type checks in the verifier.

filetests/isa/riscv/legalize-abi.cton

@@ -96,7 +96,7 @@ ebb0(v0: i64x4):
     ; check: $(v0d=$V) = iconcat $v0dl, $v0dh
     ; check: $(v0cd=$V) = vconcat $v0c, $v0d
     ; check: $(v0abcd=$V) = vconcat $v0ab, $v0cd
-    v1 = iadd v0, v0
+    v1 = bxor v0, v0
     ; check: $(v1ab=$V), $(v1cd=$V) = vsplit
     ; check: $(v1a=$V), $(v1b=$V) = vsplit $v1ab
     ; check: $(v1al=$V), $(v1ah=$V) = isplit $v1a

filetests/isa/riscv/legalize-i64.cton

@@ -57,8 +57,8 @@ ebb0(v1: i64, v2: i64):
 ; check: $(c=$V) = icmp ult, $v3l, $v1l
 ; check: [R#0c
 ; sameln: $(v3h1=$V) = iadd $v1h, $v2h
-; TODO: This doesn't typecheck. We need to convert the b1 result to i32.
+; check: $(c_int=$V) = bint.i32 $c
 ; check: [R#0c
-; sameln: $(v3h=$V) = iadd $v3h1, $c
+; sameln: $(v3h=$V) = iadd $v3h1, $c_int
 ; check: $v3 = iconcat $v3l, $v3h
 ; check: return $v3l, $v3h