Implement major new fastisel functionality: the matcher can now handle immediates with

value constraints on them (when defined as ImmLeaf's).  This is particularly important
for X86-64, where almost all reg/imm instructions take a i64immSExt32 immediate operand,
which has a value constraint.  Before this patch we ended up iseling the examples into
such amazing code as:

	movabsq	$7, %rax
	imulq	%rax, %rdi
	movq	%rdi, %rax
	ret

now we produce:

	imulq	$7, %rdi, %rax
	ret

This dramatically shrinks the generated code at -O0 on x86-64.

llvm-svn: 129691