[mlir] Require struct indices in LLVM::GEPOp to be constant

Recent commits added a possibility for indices in LLVM dialect GEP operations
to be supplied directly as constant attributes to ensure they remain such until
translation to LLVM IR happens. Make this required for indexing into LLVM
struct types to match LLVM IR requirements, otherwise the translation would
assert on constructing such IR.

For better compatibility with MLIR-style operation construction interface,
allow GEP operations to be constructed programmatically using Values pointing
to known constant operations as struct indices.

Depends On D116758

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D116759