[AMDGPU] Switch to backwards scavenging in non-spill cases

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