[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342761 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed