[InstCombine] fold a shifted bool zext to a select (2nd try)

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