Fix vector/scalar to vector/vector conversion (PR92515)

Fix vector/scalar to vector/vector conversion (PR92515)

r278235 broke conversions of vector/scalar shifts into vector/vector
shifts on targets that only provide the latter.  We need to record
whether a conversion is required in that case too.

Also, the old useless_type_conversion_p condition seemed unnecessarily
strong, since the shift amount can have a different signedness from
the shifted value and its vector type is never assumed to be identical
to vectype.  The patch therefore uses tree_nop_conversion_p instead.

2019-11-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92515
	* tree-vect-stmts.c (vectorizable_shift): Record incompatible op1
	types when converting a vector/scalar shift into a vector/vector one,
	using tree_nop_conversion_p instead of useless_type_conversion_p.
	Move the conversion code to the transform block.

From-SVN: r278295