gas: PR 25863: Fix scalar vmul inside it block when assembling for MVE

gas: PR 25863: Fix scalar vmul inside it block when assembling for MVE

This fixes PR 25863 by fixing the condition in the parsing of vmul in
do_mve_vmull.  It also simplifies the code in there fixing latent issues that
would lead to NEON code being accepted when it shouldn't.

gas/ChangeLog:
2020-05-04  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	PR gas/25863
	* config/tc-arm.c (do_mve_vmull): Fix scalar and NEON parsing of vmul.
	* testsuite/gas/arm/mve-scalar-vmult-it.d: New test.
	* testsuite/gas/arm/mve-scalar-vmult-it.s: New test.