Make `int.snegate` ignore the signedness of its operand and
unconditionally check for signed overflow like all the other `int.s*`
builtins do. Fix the prelude implementation of unary `-` for `Core.UInt`
to use `int.unegate` instead of `int.snegate`.
Fix the test for unsigned negate to actually test negating unsigned
integers, and add some tests that unary `-` also works.
We were mistakenly using the signed builtins, which produce the same
lowering for add/multiply right now, but don't for division and modulus.
When manually flipping SignedOverflowIsUB on, the signed version of add
gains the nsw (no signed wrap) flag, while the unsigned version
(correctly after this change) does not:
```
// CHECK:STDOUT: define i32 @_Cadd_i32.Main(i32 %a, i32 %b) !dbg !4 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %int.sadd = add nsw i32 %a, %b, !dbg !7
// CHECK:STDOUT: ret i32 %int.sadd, !dbg !8
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: define i32 @_Cadd_u32.Main(i32 %a, i32 %b) !dbg !9 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %int.uadd = add i32 %a, %b, !dbg !10
// CHECK:STDOUT: ret i32 %int.uadd, !dbg !11
// CHECK:STDOUT: }
```
Richard Smith
Dana Jansens