[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

llvm-svn: 311693