Fix generic shift expansion when shift amount is 0

Fix generic shift expansion when shift amount is 0

Summary:
This fixes http://llvm.org/bugs/show_bug.cgi?id=16439.

This is one possible way to approach this. The other would be to split InL>>(nbits-Amt) into (InL>>(nbits-1-Amt))>>1, which is also valid since since we only need to care about Amt up nbits-1. It's hard to tell which one is better since the shift might be expensive if this stage of expansion is not yet a legal machine integer, whereas comparisons with zero are relatively cheap at all sizes, but more expensive than a shift if the shift is on a legal machine type.

Patch by Keno Fischer!

Test Plan: regression test from http://reviews.llvm.org/D7752

Reviewers: chfast, resistor

Reviewed By: chfast, resistor

Subscribers: sanjoy, resistor, chfast, llvm-commits

Differential Revision: http://reviews.llvm.org/D4978

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235370 91177308-0d34-0410-b5e6-96231b3b80d8