[WebAssembly] FastISel: Don't fallback to SelectionDAG after BuildMI in selectCall

[WebAssembly] FastISel: Don't fallback to SelectionDAG after BuildMI in selectCall

My understanding is that once BuildMI has been called we can't fallback
to SelectionDAG.

This change moves the fallback for when getRegForValue() fails for
that target of an indirect call.  This was failing in -fPIC mode when
the callee is GlobalValue.

Add a test case that tickles this.

Differential Revision: https://reviews.llvm.org/D60908

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358793 91177308-0d34-0410-b5e6-96231b3b80d8