Fix Aarch64 bug in warning filtering.

Fix Aarch64 bug in warning filtering.

This fixes a small bug with the warning filtering code, which when a line has
generated a warning and a template decode error (due to the way templates are
resolved) which would not have been emitted and warnings are being suppressed
with -W it would erroneously emit the error.

I have no testcase for this because the only places we generate warnings during
encoding/decoding now is using msr/mrs and system registers.  They don't have a
template that would trigger this.

However an upcoming patch series will have tests in it which would expose this bug.

gas/ChangeLog:

	* config/tc-aarch64.c (output_operand_error_report): Apply filtering to
	current instead of head message.