I have manually decoded the imm field of an insertps one too many

times.  This patch causes llc and llvm-mc (which both default to
verbose-asm) to print out comments after a few common shuffle 
instructions which indicates the shuffle mask, e.g.:

	insertps	$113, %xmm3, %xmm0     ## xmm0 = zero,xmm0[1,2],xmm3[1]
	unpcklps	%xmm1, %xmm0    ## xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
	pshufd	$1, %xmm1, %xmm1        ## xmm1 = xmm1[1,0,0,0]

This is carefully factored to keep the information extraction (of the
shuffle mask) separate from the printing logic.  I plan to move the
extraction part out somewhere else at some point for other parts of
the x86 backend that want to introspect on the behavior of shuffles.

llvm-svn: 112387