[flang] Fix fir.embox codegen with constant interior shape

Fix https://github.com/flang-compiler/f18-llvm-project/issues/1416.

The `constRows` variable was being decremented too soon, causing the
last constant interior dimension extent being used to multiply the GEP
offset. This lead to wrong address computation and caused segfaults.

Note: also upstream fir.embox tests that can be upstreamed.

Differential Revision: https://reviews.llvm.org/D123130