i386: Fix up ix86_md_asm_adjust for TImode [PR98086]

i386: Fix up ix86_md_asm_adjust for TImode [PR98086]

ix86_md_asm_adjust assumes that dest_mode can be only [QHSD]Imode
and nothing else.  The patch rewrites zero-extension part to use
convert_to_mode to handle TImode and hypothetically even wider modes.

2020-12-03  Uroš Bizjak  <ubizjak@gmail.com>
	    Jakub Jelinek  <jakub@redhat.com>

gcc/
	PR target/98086
	* config/i386/i386.c (ix86_md_asm_adjustmd): Rewrite
	zero-extension part to use convert_to_mode.

gcc/testsuite/
	PR target/98086
	* gcc.target/i386/pr98086.c: New test.