[CodeGen] Store call frame size in MachineBasicBlock

Record the call frame size on entry to each basic block. This is usually
zero except when a basic block has been split in the middle of a call
sequence.

This simplifies PEI::replaceFrameIndices which previously had to visit
basic blocks in a specific order and had special handling for
unreachable blocks. More importantly it paves the way for an equally
simple implementation of a backwards version of replaceFrameIndices,
which is required to fully convert PrologEpilogInserter to backwards
register scavenging, which is preferred because it does not rely on
accurate kill flags.

Differential Revision: https://reviews.llvm.org/D156113