[WebAssembly][NFC] Rename simd-conversions test to simd-bitcasts

Summary:
This name is more accurate and I want to reuse the simd-conversions
name for testing the actual conversion ops.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D52333

llvm-svn: 342761