[ARM] Improve generation of thumb stack accesses

Currently when a stack access is out of range of an sp-relative ldr or
str then we jump straight to generating the offset with a literal pool
load or mov32 pseudo-instruction. This patch improves that in two
ways:
 * If the offset is within range of sp-relative add plus an ldr then
   use that.
 * When we use the mov32 pseudo-instruction, if putting part of the
   offset into the ldr will simplify the expansion of the mov32 then
   do so.

Differential Revision: https://reviews.llvm.org/D156875