tree-vect-generic: Tweak build_replicated_const [PR94980 2/3]

This patch makes build_replicated_const take the number of bits
in VALUE rather than calculating the width from the element type.
The callers can then use vector_element_bits to calculate the
correct element size from the vector type.

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

gcc/
	PR tree-optimization/94980
	* tree-vect-generic.c (build_replicated_const): Take the number
	of bits as a parameter, instead of the type of the elements.
	(do_plus_minus): Update accordingly, using vector_element_bits
	to calculate the correct number of bits.
	(do_negate): Likewise.