x86: Allow integer conversion without suffix in AT&T syntax

x86: Allow integer conversion without suffix in AT&T syntax

According to gas manual, suffix in instruction mnemonics isn't always
required:

When there is no sizing suffix and no (suitable) register operands to
deduce the size of memory operands, with a few exceptions and where long
operand size is possible in the first place, operand size will default
to long in 32- and 64-bit modes.

This includes cvtsi2sd, cvtsi2ss, vcvtsi2sd, vcvtsi2ss, vcvtusi2sd and
vcvtusi2ss.  Since they are used in GCC 8 and older GCC releases, they
must be allowed without suffix in AT&T syntax.

gas/

	PR gas/25622
	* testsuite/gas/i386/i386.exp: Run x86-64-default-suffix and
	x86-64-default-suffix-avx.
	* testsuite/gas/i386/noreg64.s: Remove cvtsi2sd, cvtsi2ss,
	vcvtsi2sd, vcvtsi2ss, vcvtusi2sd and vcvtusi2ss entries.
	* testsuite/gas/i386/noreg64.d: Updated.
	* testsuite/gas/i386/noreg64.l: Likewise.
	* testsuite/gas/i386/x86-64-default-suffix-avx.d: New file.
	* testsuite/gas/i386/x86-64-default-suffix.d: Likewise.
	* testsuite/gas/i386/x86-64-default-suffix.s: Likewise.

opcodes/

	PR gas/25622
	* i386-opc.tbl: Add IgnoreSize to cvtsi2sd, cvtsi2ss, vcvtsi2sd,
	vcvtsi2ss, vcvtusi2sd and vcvtusi2ss for AT&T syntax.
	* i386-tbl.h: Regenerated.