fix PR8961 - a fast isel miscompilation where we'd insert a new instruction

after sext's generated for addressing that got folded.  Previously we compiled
test5 into:

_test5:                                 ## @test5
## BB#0:
        movq    -8(%rsp), %rax          ## 8-byte Reload
        movq    (%rdi,%rax), %rdi
        addq    %rdx, %rdi
        movslq  %esi, %rax
        movq    %rax, -8(%rsp)          ## 8-byte Spill
        movq    %rdi, %rax
        ret

which is insane and wrong.  Now we produce:

_test5:                                 ## @test5
## BB#0:
	movslq	%esi, %rax
	movq	(%rdi,%rax), %rax
	addq	%rdx, %rax
	ret

llvm-svn: 123414