X86RegisterInfo: eliminateFrameIndex: Avoid code duplication; NFC

X86RegisterInfo: eliminateFrameIndex: Avoid code duplication; NFC

X86RegisterInfo::eliminateFrameIndex() and
X86FrameLowering::getFrameIndexReference() both had logic to compute the
base register. This consolidates the code.

Also use MachineInstr::isReturn instead of manually enumerating tail
call instructions (return instructions were not included in the previous
list because they never reference frame indexes).

Differential Revision: https://reviews.llvm.org/D32206

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300923 91177308-0d34-0410-b5e6-96231b3b80d8