Legalize bnot using xor with -1.

cranelift/filetests/isa/riscv/expand-i32.cton

@@ -32,7 +32,8 @@ ebb0(v0: i32):
 function %bitclear(i32, i32) -> i32 {
 ebb0(v0: i32, v1: i32):
     v2 = band_not v0, v1
-    ; check: bnot
+    ; check: iconst.i32 -1
+    ; check: bxor
     ; check: band
     return v2
 }

lib/codegen/meta/base/legalize.py

@@ -252,6 +252,14 @@ for inst_not,      inst in [
                 a << inst(x, a1)
             ))
 
+# Expand bnot using xor.
+expand.legalize(
+        a << bnot(x),
+        Rtl(
+            y << iconst(imm64(-1)),
+            a << bxor(x, y)
+        ))
+
 # Floating-point sign manipulations.
 for ty,             minus_zero in [
         (types.f32, f32const(ieee32.bits(0x80000000))),