Use literal pool loads instead of MOVW/MOVT for materializing global addresses...

Use literal pool loads instead of MOVW/MOVT for materializing global addresses when optimizing for size.

On spec/gcc, this caused a codesize improvement of ~1.9% for ARM mode and ~4.9% for Thumb(2) mode. This is
codesize including literal pools.

The pools themselves doubled in size for ARM mode and quintupled for Thumb mode, leaving suggestion that there
is still perhaps redundancy in LLVM's use of constant pools that could be decreased by sharing entries.

Fixes PR11087.

llvm-svn: 142530