[Fix][TIRx] Handle vector access pointer addresses in C codegen

[tlopex]

This PR fixes invalid pointer arithmetic emitted by C-family codegen for vector-typed tvm_access_ptr.

A vector access pointer is lowered to address_of(BufferLoad(...)) with a Ramp index describing its lane indices. For example, Ramp(4, 1, 2) represents scalar elements [4, 5], so its address should be the address of the first lane, &A[4].

LLVM codegen already extracts Ramp::base in this case. However, CodeGenC previously passed the complete ramp to pointer arithmetic, which could generate invalid CUDA code such as:

(float*)A + make_int2(4, 5)

This PR makes CodeGenC use Ramp::base when generating the address of a vector BufferLoad. The normalized index is applied to both the direct pointer-offset path and the general GetBufferRef path.

The existing scalar-buffer plus Ramp lowering is preserved. This avoids regressions for padded vector types such as float32x3 and packed vector types such as int4x4, while making C-family codegen consistent with LLVM codegen.

Regression tests cover:

• float32x2 C codegen.
• Padded float32x3 LLVM codegen.
• Packed int4x4 CUDA codegen.