Record the vector mask precision in stmt_vec_info

Record the vector mask precision in stmt_vec_info

search_type_for_mask uses a worklist to search a chain of boolean
operations for a natural vector mask type.  This patch instead does
that in vect_determine_stmt_precisions, where we also look for
overpromoted integer operations.  We then only need to compute
the precision once and can cache it in the stmt_vec_info.

The new function vect_determine_mask_precision is supposed
to handle exactly the same cases as search_type_for_mask_1,
and in the same way.  There's a lot we could improve here,
but that's not stage 3 material.

I wondered about sharing mask_precision with other fields like
operation_precision, but in the end that seemed too dangerous.
We have patterns to convert between boolean and non-boolean
operations and it would be very easy to get mixed up about
which case the fields are describing.

2019-11-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (stmt_vec_info::mask_precision): New field.
	(vect_use_mask_type_p): New function.
	* tree-vect-patterns.c (vect_init_pattern_stmt): Copy the
	mask precision to the pattern statement.
	(append_pattern_def_seq): Add a scalar_type_for_mask parameter
	and use it to initialize the new stmt's mask precision.
	(search_type_for_mask_1): Delete.
	(search_type_for_mask): Replace with...
	(integer_type_for_mask): ...this new function.  Use the information
	cached in the stmt_vec_info.
	(vect_recog_bool_pattern): Update accordingly.
	(build_mask_conversion): Pass the scalar type associated with the
	mask type to append_pattern_def_seq.
	(vect_recog_mask_conversion_pattern): Likewise.  Call
	integer_type_for_mask instead of search_type_for_mask.
	(vect_convert_mask_for_vectype): Call integer_type_for_mask instead
	of search_type_for_mask.
	(possible_vector_mask_operation_p): New function.
	(vect_determine_mask_precision): Likewise.
	(vect_determine_stmt_precisions): Call it.

From-SVN: r278850