When the scavenger is not allowed to spill, the only difference between
forward and backward should be the heuristics used to pick an available
register. Forwards scavenging tries to pick a register that can be used
again later in the BB; backwards scavenging tries to pick one that can
be used earlier.
Backwards scavenging is preferred because it does not rely on accurate
kill flags.
Differential Revision: https://reviews.llvm.org/D151323
94e48d433d