[ARM] Register pressure with -mthumb forces register reload before each call

This patch implements the foldMemoryOperand hook in Thumb1InstrInfo,
allowing tBLXr and a spilled function address to be combined back into a
tBL. This can help with codesize at Oz, especailly in the tinycrypt
library.

Differential Revision: https://reviews.llvm.org/D79785