[WebAssembly] FastISel : Bail to SelectionDAG for constexpr calls

Summary: Currently FastISel lowers constexpr calls as indirect calls.
We'd like those to direct calls, and falling back to SelectionDAGISel
handles that.

Reviewers: dschuff, sunfish

Subscribers: jfb, sbc100, llvm-commits, aheejin

Differential Revision: https://reviews.llvm.org/D37073

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311693 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed