tests: update check for inferred nneg on zext

This was broken by upstream
llvm/llvm-project@dc6d077. It's easy
enough to use a regex match to support both, so we do that.

r? @nikic
@rustbot label: +llvm-main

tests/codegen/unchecked_shifts.rs

@@ -31,7 +31,7 @@ pub unsafe fn unchecked_shl_unsigned_smaller(a: u16, b: u32) -> u16 {
 #[no_mangle]
 pub unsafe fn unchecked_shl_unsigned_bigger(a: u64, b: u32) -> u64 {
  // CHECK-NOT: assume
- // CHECK: %[[EXT:.+]] = zext i32 %b to i64
+ // CHECK: %[[EXT:.+]] = zext{{( nneg)?}} i32 %b to i64
  // CHECK: shl i64 %a, %[[EXT]]
  a.unchecked_shl(b)
 }
@@ -63,7 +63,7 @@ pub unsafe fn unchecked_shr_signed_smaller(a: i16, b: u32) -> i16 {
 #[no_mangle]
 pub unsafe fn unchecked_shr_signed_bigger(a: i64, b: u32) -> i64 {
  // CHECK-NOT: assume
- // CHECK: %[[EXT:.+]] = zext i32 %b to i64
+ // CHECK: %[[EXT:.+]] = zext{{( nneg)?}} i32 %b to i64
  // CHECK: ashr i64 %a, %[[EXT]]
  a.unchecked_shr(b)
 }