[ARM][CGP] Allow signext arguments

As we perform a zext on any arguments used in the promoted tree, it
doesn't matter if they're marked as signext. The only permitted
user(s) in the tree which would interpret the sign bits are signed
icmps. For these instructions, their promoted operands are truncated
before the icmp uses them.

Differential Revision: https://reviews.llvm.org/D68019

llvm-svn: 373186