[CIR][CodeGen] Support VLA pointer arithmetic (#196837)
Implement pointer arithmetic on variable-length array types in CIR
codegen, removing two NYI diagnostics:
1. Binary pointer arithmetic (p + i, p - i) where the pointee is a VLA:
multiply the index by the runtime VLA size before emitting PtrStrideOp.
2. Unary increment/decrement (p++, p--) on pointers to VLA types: stride
by the VLA element count, negating for decrement.
Fixes #192318
