[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