Arm: Fix IT-predicated MVE vcvt

This patch fixes an incorrect parsing of vcvtne for MVE. Even when it appeared
in an IT block, vcvtne would be parsed as a VPT-predicated vcvtn, instead of an
IT-predicated vcvt. This change extends the existing handling of MVE vcvt to
properly account for IT predication.

gas/ChangeLog:

2020-08-13  Joe Ramsay  <joe.ramsay@arm.com>

	Backported from master
	2020-08-04  Joe Ramsay <joe.ramsay@.arm.com>

	* config/tc-arm.c (do_neon_cvt_1): Parse vcvtne as vcvt-ne for
	NS_FD shape when MVE is present
	* testsuite/gas/arm/mve-vcvtne-it-bad.d: New test.
	* testsuite/gas/arm/mve-vcvtne-it-bad.l: New test.
	* testsuite/gas/arm/mve-vcvtne-it-bad.s: New test.
	* testsuite/gas/arm/mve-vcvtne-it.d: New test.
	* testsuite/gas/arm/mve-vcvtne-it.s: New test.