[InstCombine] Simplify funnel shift with zero/undef operand to shift

The following simplifications are implemented:

 * `fshl(X, 0, C) -> shl X, C%BW`
 * `fshl(X, undef, C) -> shl X, C%BW` (assuming undef = 0)
 * `fshl(0, X, C) -> lshr X, BW-C%BW`
 * `fshl(undef, X, C) -> lshr X, BW-C%BW` (assuming undef = 0)
 * `fshr(X, 0, C) -> shl X, (BW-C%BW)`
 * `fshr(X, undef, C) -> shl X, BW-C%BW` (assuming undef = 0)
 * `fshr(0, X, C) -> lshr X, C%BW`
 * `fshr(undef, X, C) -> lshr, X, C%BW` (assuming undef = 0)

The simplification is only performed if the shift amount C is constant,
because we can explicitly compute C%BW and BW-C%BW in this case.

Differential Revision: https://reviews.llvm.org/D54778

llvm-svn: 347505