[WebAssembly] Standardize order of SIMD bitselect arguments

For some reason the backend assumed that the condition mask would be
the first argument to the LLVM intrinsic, but everywhere else the
condition mask is the third argument.

Reviewers: aheejin

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

Differential Revision: https://reviews.llvm.org/D56412

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350746 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed