[ARM] Mark LEApcrel instructions as isAsCheapAsAMove

Doing this means that if an LEApcrel is used in two places we will rematerialize
instead of generating two MOVs. This is particularly useful for printfs using
the same format string, where we want to generate an address into a register
that's going to get corrupted by the call.

Differential Revision: https://reviews.llvm.org/D32858

llvm-svn: 303054