fix an x86 fast isel issue where we'd completely give up on folding an address

when we have a global variable base an an index.  Instead, just give up on
folding the global variable.

Before we'd geenrate:

_test:                                  ## @test
## BB#0:
	movq	_rtx_length@GOTPCREL(%rip), %rax
	leaq	(%rax), %rax
	addq	%rdi, %rax
	movzbl	(%rax), %eax
	ret

now we generate:

_test:                                  ## @test
## BB#0:
	movq	_rtx_length@GOTPCREL(%rip), %rax
	movzbl	(%rax,%rdi), %eax
	ret

The difference is even more significant when there is a scale
involved.

This fixes rdar://9289558 - total fail with addr mode formation at -O0/x86-64

llvm-svn: 129664