[Patch] The simplest optimisation! (MOVSXD)

Original Reporter info from Mantis: CuriousKit @CuriousKit

Reporter name: J. Gareth Moreton

Description:

This patch, for x86_64, includes the 64-bit exclusive MOVSXD instruction, alongside MOVZX and MOVSX, for analysis through the OptPass1Movx routine.

Steps to reproduce:

Apply patch and confirm correct compilation.

Additional information:

The inclusion of the instruction improves some instruction sequences that deal with signed values in memory being sign-extended to the CPU word size (as the language is defined to do) before being stored again after some basic operation - for example:

	movslq	-360(%rbp),%rax
	addq	%rdx,%rax
	movq	%rax,-360(%rbp)

Becomes:

addq	%rdx,-360(%rbp)

And:

	movslq	-36(%rbp),%rax
	orq		$34816,%rax
	movl	%eax,-36(%rbp)