[Bug rtl-optimization/118873] -favoid-store-forwarding makes a mess out of a STLF fail

[konstantinos.eleftheriou at vrull dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118873

--- Comment #1 from Konstantinos Eleftheriou <konstantinos.eleftheriou at vrull dot eu> ---
These subregs are generated from `store_bit_field`. We could fix this by
updating `store_bit_field` or avoid calling it in those cases and handle this
in-place. Actually, we're rejecting cases with scalar-vector store-loads for
the same reason.