[LSR] Improve filtered uses in NarrowSearchSpaceByPickingWinnerRegs

[LSR] Improve filtered uses in NarrowSearchSpaceByPickingWinnerRegs

NarrowSearchSpaceByPickingWinnerRegs has an aggressive filtering method to
reduce the complexity of the search space down by picking a best formula with
the highest number of reuses and assuming it will yield profitable reuse. In
certain cases we can find a best formula like {X+30,+,1} and later check a
formula like {X,+,1} with the same number of Uses. On some architectures it
can be better to pick {X,+,1}, especially if an offset of 30 can be used as a
legal addressing mode, but -30 cannot. That happens under Thumb1 code, which
has fairly limited addressing modes. This patch adds a check to see if it can
pick the simpler formula, if it looks more profitable.

Differential Revision: https://reviews.llvm.org/D144014