[flang] convert stack arrays allocation to match old type

The old fir.allocmem operation returned a !fir.heap<.> type. The new
fir.alloca operation returns a !fir.ref<.> type. This patch inserts a
fir.convert so that the old type is preserved. This prevents verifier
failures when types returned from fir.if statements don't match the
expected type.

Differential Revision: https://reviews.llvm.org/D151921