[ARM] Use preferred alignment for constants in promoteToConstantPool.

This mostly affects IR generated by non-clang frontends because clang
generally sets the alignment of globals explicitly.

Fixes https://bugs.llvm.org//show_bug.cgi?id=32394 .

(-arm-promote-constant is currently off by default, and it stays off
with this patch. I'll look into turning it on again when all the known
issues are fixed.)

Differential Revision: https://reviews.llvm.org/D51469

llvm-svn: 343359