[CodeGen][inlineasm] assume the flag output of inline asm is boolean value
GCC inline asm document says that
"... the general rule is that the output variable must be a scalar
integer, and the value is boolean."
Commit
e5c37958f901cc9bec50624dbee85d40143e4bca lowers flag output of
inline asm on X86 with setcc, hence it is guaranteed that the flag
is of boolean value. Clang does not support ARM inline asm flag output
yet so nothing need to be worried about ARM.
See "Flag Output" section at
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#OutputOperands
Fixes https://github.com/llvm/llvm-project/issues/56568
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D129954