[fir] Add !fir.alloca conversion

Convert !fir.alloca into !llvm.alloca.

This also contains a fix for verifyInType in FIROps.cpp, to make sure we
can handle a !fir.ptr<!fir.array<?xN>>.

Differential Revision: https://reviews.llvm.org/D113563



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>