[CodeGen] Store SP adjustment in MachineBasicBlock. NFCI.

Record the SP adjustment on entry to each basic block. This is almost
always zero except on targets like ARM which can split a basic block 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/D154281