[ARM][CGP] Guard against signext args and sitofp

Add an Argument that has the SExtAttr attached, as well as SIToFP
instructions, as values that generate sign bits. SIToFP doesn't
strictly do this and could be treated as a sink to be sign-extended.

Differential Revision: https://reviews.llvm.org/D61381

llvm-svn: 360331