[ARM] Fix correctness checks in promoteToConstantPool.

Correctly check for relocations in the constant to promote. And don't
allow promoting a constant multiple times.

This partially fixes https://bugs.llvm.org//show_bug.cgi?id=32780 ;
it's not a complete fix because we also need to prevent
ARMConstantIslands from cloning the constant.

(-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/D51472

llvm-svn: 343361