i386: Fix ix86_fold_builtin shift folding [PR93418]

i386: Fix ix86_fold_builtin shift folding [PR93418]

The following testcase is miscompiled, because the variable shift left
operand, { -1, -1, -1, -1 } is represented as a VECTOR_CST with
VECTOR_CST_NPATTERNS 1 and VECTOR_CST_NELTS_PER_PATTERN 1, so when
we call builder.new_unary_operation, builder.encoded_nelts () will be just 1
and thus we encode the resulting vector as if all the elements were the
same.
For non-masked is_vshift, we could perhaps call builder.new_binary_operation
(TREE_TYPE (args[0]), args[0], args[1], false), but then there are masked
shifts, for non-is_vshift we could perhaps call it too but with args[2]
instead of args[1], but there is no builder.new_ternary_operation.
All this stuff is primarily for aarch64 anyway, on x86 we don't have any
variable length vectors, and it is not a big deal to compute all elements
and just let builder.finalize () find the most efficient VECTOR_CST
representation of the vector.  So, instead of doing too much, this just
keeps using new_unary_operation only if only one VECTOR_CST is involved
(i.e. non-masked shift by constant) and for the rest just compute all elts.

2020-01-28  Jakub Jelinek  <jakub@redhat.com>

	PR target/93418
	* config/i386/i386.c (ix86_fold_builtin) <do_shift>: If mask is not
	-1 or is_vshift is true, use new_vector with number of elts npatterns
	rather than new_unary_operation.

	* gcc.target/i386/avx2-pr93418.c: New test.