[SLP][VECT] Add check to fix 96837

[SLP][VECT] Add check to fix 96837

The following patch adds a simple check to prevent slp stmts from
vector constructors being rearranged. vect_attempt_slp_rearrange_stmts
tries to rearrange to avoid a load permutation.

This fixes PR target/96837
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96827

gcc/ChangeLog:

2020-09-29  Joel Hutton  <joel.hutton@arm.com>

	PR target/96837
	* tree-vect-slp.c (vect_analyze_slp): Do not call
	vect_attempt_slp_rearrange_stmts for vector constructors.

gcc/testsuite/ChangeLog:

2020-09-29  Joel Hutton  <joel.hutton@arm.com>

	PR target/96837
	* gcc.dg/vect/bb-slp-49.c: New test.