[InstCombine] Teach SimplifyDemandedUseBits to shrink Constants on the left side of subtracts

Summary: Subtracts can have constants on the left side, but we don't shrink them based on demanded bits. This patch fixes that to match the right hand side.

Reviewers: davide, majnemer, spatel, sanjoy, hfinkel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31119

llvm-svn: 298478