[ARM] Support ldr.w in pseudo instruction ldr rd,=immediate

The changes made in r269352, r269353 and r269354 to support the 
transformation of the ldr rd,=immediate to mov introduced a regression
from 3.8 (ldr.w rd, =immediate) not supported.

This change puts support back in for ldr.w by means of a t2InstAlias for
the .w form. The .w is ignored in ARM state and propagated to the ldr in
Thumb2.

llvm-svn: 281319