[Thumb] Fix assembler error 'cannot honor width suffix pop {lr}'

Summary:
* avoid generating POP {LR} in Thumb1 epilogues
* combine MOV LR, Rx + BX LR -> BX Rx in a peephole optimization pass
* combine POP {LR} + B + BX LR -> POP {PC} on v5T+

Test cases by Ana Pazos

Differential Revision: http://reviews.llvm.org/D15707

llvm-svn: 256523