[flang][FIR] add fir.assumed_size_extent to abstract assumed-size extent encoding (#164452)

The purpose of this patch is to allow converting FIR array representation to
memref when possible without hitting memref verifier issue.

The issue was that FIR arrays may be assumed size, in which case the
last dimension will not be known at runtime. Flang uses -1 to encode
this to fulfill Fortran 2023 standard requirements in 18.5.3 point 5
about CFI_desc_t.

When arrays are converted to memeref, if this `-1` reaches memeref
operations, it triggers verifier errors (even if the conversion happened
in code that guards the code to be entered at runtime if the array is
assumed-size because folders/verifiers do not take into account
reachability).

This follows-up on discussions in #163505 merge requests

0 files changed, 0 insertions, 0 deletions