[GlobalIsel][X86] Legalize G_BSWAP

remark: unable to legalize instruction: %95:_(s16) = G_BSWAP %94:_ (in function: _ZNK4llvm13DataExtractor6getU16EPyPtj) [-Rpass-missed=gisel-legalize]

check plan: ninja check-llvm-codegen-x86

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D150667