InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add.

This was intended to undo the sub canonicalization in cases where it's not profitable, but it also
finds some cases on it's own.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147256 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstCombine/sub-xor.ll

@@ -23,3 +23,15 @@ define i32 @test2(i32 %x) nounwind {
 ; CHECK-NEXT: xor i32 %count, 31
 ; CHECK-NEXT: ret
 }
+
+define i32 @test3(i32 %x) nounwind {
+  %and = and i32 %x, 31
+  %sub = xor i32 31, %and
+  %add = add i32 %sub, 42
+  ret i32 %add
+
+; CHECK: @test3
+; CHECK-NEXT: and i32 %x, 31
+; CHECK-NEXT: sub i32 73, %and
+; CHECK-NEXT: ret
+}