[ARM] Allow signed icmps in ARMCodeGenPrepare

Treat signed icmps as 'sinks', allowing them to be in the use-def
tree, enabling more promotions to be performed. As a sink, any
promoted incoming values need to be truncated before being used by
the signed icmp.

Differential Revision: https://reviews.llvm.org/D50067

llvm-svn: 339755