[Thumb1] Fix cost calculation for complemented immediates

Materializing something like "-3" can be done as 2 instructions:
  MOV r0, #3
  MVN r0, r0

This has a cost of 2, not 3. It looks like we were already trying to detect this pattern in TII::getIntImmCost(), but were taking the complement of the zero-extended value instead of the sign-extended value which is unlikely to ever produce a number < 256.

There were no tests failing after changing this... :/

llvm-svn: 280928