Fix some scavenger performance issues.

- Don't call malloc+free in the very hot forward().
- Don't call isTiedToDefOperand().
- Don't create BitVector temporaries.
- Merge DeadRegs into KillRegs.
- Eliminate the early clobber checks, they were irrelevant to scavenging.
- Remove unnecessary code from -Asserts builds.

This speeds up ARM PEI by 3.4x and overall llc -O0 codegen time by 11%.

llvm-svn: 149189