[WebAssembly] Fix bug in custom shuffle combine

Summary:
The code previously assumed the source of the bitcast in the combined
pattern was a vector type, but this is not always true. This patch
adds a check to avoid an assertion failure in that case.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80164