MachineCopyPropagation: Keep scanning through instructions with regmasks

This also simplifies the code by removing the overly conservative
NoInterveningSideEffect() function. This function checked:
- That the two copies belong to the same block: We only process one
  block at a time and clear our maps in between it is impossible to find a
  copy from a different block.
- There is no terminator between the two copy instructions: This is not
  allowed anyway (the MachineVerifier would complain)
- Does not have instructions with hasUnmodeledSideEffects() or isCall()
  set: Even for those instructuction we must have all clobbers/defs of
  registers explicit as an operand. If the register is explicitely
  clobbered we would never come to the point of checking for
  NoInterveningSideEffect() anyway.

(I also checked this with a temporary build of the test-suite with all
 potentially failing conditions in NoInterveningSideEffect() turned into
 asserts)

Differential Revision: http://reviews.llvm.org/D17474

llvm-svn: 261965