middle-end: [PR middle-end/116926] Allow widening optabs for vec-mode -> scalar-mode

The recent refactoring of the dot_prod optab to convert-type exposed a
limitation in how `find_widening_optab_handler_and_mode' is currently
implemented, owing to the fact that, while the function expects the

  GET_MODE_CLASS (from_mode) == GET_MODE_CLASS (to_mode)

condition to hold, the c6x backend implements a dot product from V2HI
to SI, which triggers an ICE.

Consequently, this patch adds some logic to allow widening optabs
which accumulate vector elements to a single scalar.

gcc/ChangeLog:

	PR middle-end/116926
	* optabs-query.cc (find_widening_optab_handler_and_mode): Add
	handling of vector -> scalar optab handling.