[X86ISelLowering] permit BlockAddressSDNode "i" constraints for PIC

When building 32b x86 code as PIC, the existing handling of "i"
constraints is conservative since generally we have to go through the
GOT to find references to functions.

But generally, BlockAddresses from C code refer to the Function in the
current TU.  Permit BlockAddresses to be used with the "i" constraint
for those cases.

I regressed this in
commit 4edb9983cb8c ("[SelectionDAG] treat X constrained labels as i for asm")

Fixes: https://github.com/llvm/llvm-project/issues/53868

Reviewed By: efriedma, MaskRay

Differential Revision: https://reviews.llvm.org/D119905