Remove variable that keeps the size of area used to save byval or variable

Remove variable that keeps the size of area used to save byval or variable
argument registers on the callee's stack frame, along with functions that set
and get it.

It is not necessary to add the size of this area when computing stack size in
emitPrologue, since it has already been accounted for in
PEI::calculateFrameObjectOffsets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144549 91177308-0d34-0410-b5e6-96231b3b80d8