[X86][MC] Make -x86-pad-max-prefix-size compatible with --mc-relax-all
Summary: We allow non-relaxable instructions emitted into relaxable Fragment when we prefix padding branch. So we need to check if the instruction need relaxation before relaxing it. Without this patch, it currently triggers a report_fatal_error in llvm::MCAsmBackend::relaxInstruction when we prefix padding branch along with --mc-relax-all.
Reviewers: LuoYuanke, reames, MaskRay
Reviewed By: MaskRay
Subscribers: MaskRay, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D77851