[ARM] Avoid injecting constant islands in movw+movt pairs on Windows

On Windows, movw+movt pairs with relocations are handled with a single
relocation that covers them both. Therefore we can't inject anything
between these instructions, otherwise the relocation (which in LLVM
only is treated as the movw instruction's relocation, while the movt
instruction's relocation is dropped) will end up bogus.

These instructions are bundled up until right before the constant
islands pass, making this effectively the only place that can split
them apart.

Differential Revision: https://reviews.llvm.org/D51032

llvm-svn: 340451