[WebAssembly][NFC] Simplify vector shift lowering and add tests

This patch builds on 0d7286a6 by simplifying the code for detecting
splat values and adding new tests demonstrating the lowering of
splatted absolute value shift amounts, which are common in code
generated by Halide. The lowering is very bad right now, but
subsequent patches will improve it considerably. The tests will be
useful for evaluating the improvements in those patches.

Reviewed By: aheejin

Differential Revision: https://reviews.llvm.org/D83493