In SDISel, for targets that support FORMAL_ARGUMENTS nodes, lower this

node as soon as we create it in SDISel.  Previously we would lower it in
legalize.  The problem with this is that it only exposes the argument
loads implied by FORMAL_ARGUMENTs after legalize, so that only dag combine 2
can hack on them.  This causes us to miss some optimizations because 
datatype expansion also happens here.

Exposing the loads early allows us to do optimizations on them.  For example
we now compile arg-cast.ll to:

_foo:
	movl	$2147483647, %eax
	andl	8(%esp), %eax
	ret

where we previously produced:

_foo:
	subl	$12, %esp
	movsd	16(%esp), %xmm0
	movsd	%xmm0, (%esp)
	movl	$2147483647, %eax
	andl	4(%esp), %eax
	addl	$12, %esp
	ret

It might also make sense to do this for ISD::CALL nodes, which have implicit
stores on many targets.

llvm-svn: 47054