[ARM] Use lrdsb for more thumb1 loads.

Given a sextload i16, we can usually generate "ldrsh [rn. rm]". If we
don't naturally have a rn, rm addressing mode, we can either generate
"ldrh [rn, #0]; sxth" or "mov rm, #0; ldrsh [rn. rm]".

We currently generate the first, always creating a sxth. They are both
the same number of instructions, but if we generate the second then the
mov #0 will likely be CSE'd or pulled out of a loop, etc.

This adjusts the ISel patterns to do that, creating a mov instead of a
sxth.

Differential Revision: https://reviews.llvm.org/D98693