[TargetLowering] Handle multi depth GEPs w/ inline asm constraints

Summary:
X86TargetLowering::LowerAsmOperandForConstraint had better support than
TargetLowering::LowerAsmOperandForConstraint for arbitrary depth
getelementpointers for "i", "n", and "s" extended inline assembly
constraints. Hoist its support from the derived class into the base
class.

Link: https://github.com/ClangBuiltLinux/linux/issues/469

Reviewers: echristo, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, E5ten, kees, jyknight, nemanjai, javed.absar, eraman, hiraditya, jsji, llvm-commits, void, craig.topper, nathanchance, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61560

llvm-svn: 360604