[TargetLowering] support BlockAddress as "i" inline asm constraint

Summary:
This allows passing address of labels to inline assembly "i" input
constraints.

Fixes pr/42502.

Reviewers: ostannard

Reviewed By: ostannard

Subscribers: void, echristo, nathanchance, ostannard, javed.absar, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64167

llvm-svn: 365664