[WebAssembly] Select call_indirect for alloca calls

Currently calling stack locations is selected using `CALL` in ISel,
resulting in an invalid code and crashing in AsmPrinter. FastISel
correctly selects it will `CALL_INDIRECT`.

Fixes the problem reported in D146781.

Reviewed By: tlively, HerrCai0907

Differential Revision: https://reviews.llvm.org/D147033