[ARM] Allow pointer values in ARMCodeGenPrepare

Add pointers to the list of allowed types, but don't try to promote
them. Also fixed a bug with the promotion of undef values, so a new
value is now created instead of mutating in place. We also now only
promote if there's an instruction in the use-def chains other than
the icmp, sinks and sources.

Differential Revision: https://reviews.llvm.org/D50054

llvm-svn: 339754