Add bitwise ops that invert the second operand.

ARM has all of these as scalar integer instructions. Intel has band_not
in SSE and as a scalar in BMI1.

Add the trivial legalization patterns that use a bnot instruction.

filetests/isa/riscv/expand-i32.cton

@@ -28,3 +28,11 @@ ebb0(v0: i32):
 ; check: $(cst=$V) = iconst.i32 0x3b9a_ca00
 ; check: $v1 = iadd $v0, $cst
 ; check: return $v1
+
+function %bitclear(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = band_not v0, v1
+    ; check: bnot
+    ; check: band
+    return v2
+}