[VE] Optimize prologue/epilogue instructions
Optimize eliminate FP mechanism. This time optimize a function which has
no call but fixed stack objects. LLVM eliminates FP on such functions now.
Also, optimize GOT/PLT registers save/restore instructions if a given
function doesn't uses them. In addition, remove generating mechanism of
.cfi instructions since those are taken from other architectures and not
inspected yet. Update regression tests, also.
Reviewed By: simoll
Differential Revision: https://reviews.llvm.org/D92251