MIPS/GAS: Also respect `-mignore-branch-isa' with MIPS16 code

Fix a bug in commit 8b10b0b3 ("MIPS: Add options to control branch
ISA checks") and with the `-mignore-branch-isa' command-line option also
lift a GAS check for invalid MIPS16 branches between ISA modes, which is
made separately from regular MIPS and microMIPS checks.

	gas/
	* config/tc-mips.c (md_convert_frag): Respect
	`mips_ignore_branch_isa'.
	* testsuite/gas/mips/branch-local-5.d: New test.
	* testsuite/gas/mips/branch-local-n32-5.d: New test.
	* testsuite/gas/mips/branch-local-n64-5.d: New test.
	* testsuite/gas/mips/branch-local-6.d: New test.
	* testsuite/gas/mips/branch-local-n32-6.d: New test.
	* testsuite/gas/mips/branch-local-n64-6.d: New test.
	* testsuite/gas/mips/branch-local-7.d: New test.
	* testsuite/gas/mips/branch-local-n32-7.d: New test.
	* testsuite/gas/mips/branch-local-n64-7.d: New test.
	* testsuite/gas/mips/branch-local-ignore-5.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n32-5.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n64-5.d: New test.
	* testsuite/gas/mips/branch-local-ignore-6.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n32-6.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n64-6.d: New test.
	* testsuite/gas/mips/branch-local-5.l: New stderr output.
	* testsuite/gas/mips/branch-local-6.l: New stderr output.
	* testsuite/gas/mips/branch-local-5.s: New test source.
	* testsuite/gas/mips/branch-local-6.s: New test source.
	* testsuite/gas/mips/branch-local-7.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.

(cherry picked from commit 37b2d327)