The 1st attempt at rL374828 inserted the code
at the wrong position (outside of the constant-shift-amount
block). Trying again with an additional test to verify
const-ness.
For a constant shift amount, add the following fold.
shl (zext (i1 X)), ShAmt --> select (X, 1 << ShAmt, 0)
https://rise4fun.com/Alive/IZ9
Fixes PR42257.
Based on original patch by @zvi (Zvi Rackover)
Differential Revision: https://reviews.llvm.org/D63382
llvm-svn: 374886