[GlobalISel][Legalizer] Widening the second src op of shifts bug fix

The second source operand of G_SHL, G_ASHR, and G_LSHR must preserve its
value as a (small) unsigned integer, therefore its incorrect to widen it
in any way but by zero extending it.

G_SHL was using G_ANYEXT and G_ASHR - G_SEXT (which is correct for their
destination and first source operands, but not the "number of bits to
shift" operand).

Generally, shifts aren't as similar to regular binary operations as it
might seem, for instance, they aren't commutative nor associative and
the second source operand usually requires a special treatment.

Reviewers: bogner, javed.absar, aivchenk, rovka

Reviewed By: bogner

Subscribers: igorb, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D46413

llvm-svn: 331926