tree: Add vector_element_bits(_tree) [PR94980 1/3]

A lot of code that wants to know the number of bits in a vector
element gets that information from the element's TYPE_SIZE,
which is always equal to TYPE_SIZE_UNIT * BITS_PER_UNIT.
This doesn't work for SVE and AVX512-style packed boolean vectors,
where several elements can occupy a single byte.

This patch introduces a new pair of helpers for getting the true
(possibly sub-byte) size.  I made a token attempt to convert obvious
element size calculations, but I'm sure I missed some.

2020-05-12  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/94980
	* tree.h (vector_element_bits, vector_element_bits_tree): Declare.
	* tree.c (vector_element_bits, vector_element_bits_tree): New.
	* match.pd: Use the new functions instead of determining the
	vector element size directly from TYPE_SIZE(_UNIT).
	* tree-vect-data-refs.c (vect_gather_scatter_fn_p): Likewise.
	* tree-vect-patterns.c (vect_recog_mask_conversion_pattern): Likewise.
	* tree-vect-stmts.c (vect_is_simple_cond): Likewise.
	* tree-vect-generic.c (expand_vector_piecewise): Likewise.
	(expand_vector_conversion): Likewise.
	(expand_vector_addition): Likewise for a TYPE_SIZE_UNIT used as
	a divisor.  Convert the dividend to bits to compensate.
	* tree-vect-loop.c (vectorizable_live_operation): Call
	vector_element_bits instead of open-coding it.