[WebAssembly] Fix lowering of varargs functions with non-legal fixed arguments.

CallLoweringInfo's NumFixedArgs field gives the number of fixed arguments
before legalization. The ISD::OutputArg "Outs" array holds legalized
arguments, so when indexing into it to find the non-fixed arguemn, we need
to use the number of arguments after legalization.

Fixes PR37934.

llvm-svn: 335576