[flang] do not merge block after lowering

Lowering relies on dead code generation / unreachable block deletion
to delete some code that is potentially invalid.

However, calling mlir::simplifyRegion also merges block, which may
promote SSA values to block arguments. Not all FIR types are intended
to be block arguments.
The added test shows an example where block merging led to
fir.shape<> being block arguments (and a failure later in codegen).

Reviewed By: tblah, clementval, vdonaldson

Differential Revision: https://reviews.llvm.org/D153858