[CIR][Lowering][Bugfix] Fix GetMemberOp lowering (llvm#273)

The wrong element type was being passed to LLVM's GEP op, generating an
invalid IR. Tests were also updated to properly validate the
`llvm.getelementptr` element type.

Fixes llvm#272

Author: sitio-couto

clang/test/CIR/Lowering/struct.cir

@@ -1,6 +1,5 @@
 // RUN: cir-opt %s -cir-to-llvm -o %t.mlir
 // RUN: FileCheck --input-file=%t.mlir %s
-// XFAIL: *
 
 !s32i = !cir.int<s, 32>
 !u8i = !cir.int<u, 8>