[Bug middle-end/50481] builtin to reverse the bit order

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

--- Comment #42 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
The LoongArch bitreversehi2 somehow evades (maybe happens to evade?) the extra
extension.  It performs a bitreverse operation in full length GPR (i.e. 64-bit
bitrev.d is used instead of 32-bit bitrev.w) first, then shifts the result, and
finally sets SRP_UNSIGNED.