[WebAssembly] Fix SIMD shift unrolling to avoid assertion failure

Summary:
Using the default DAG.UnrollVectorOp on v16i8 and v8i16 vectors
results in i8 or i16 nodes being inserted into the SelectionDAG. Since
those are illegal types, this causes a legalization assertion failure
for some code patterns, as uncovered by PR45178. This change unrolls
shifts manually to avoid this issue by adding and using a new optional
EVT argument to DAG.ExtractVectorElements to control the type of the
extract_element nodes.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76043