Fixes an ICE in the parser, and deprecates a previously allowed style of
syntax that deviated from GNU-style extended asm.

Reviewed-on: https://github.com/dlang/dmd/pull/10916

gcc/testsuite/ChangeLog:

2020-03-16  Iain Buclaw  <ibuclaw@gdcproject.org>

	* gdc.dg/asm1.d: Add new test for ICE in asm parser.
	* gdc.dg/asm5.d: New test.