[TBAA] Use !tbaa for first accessed field if it is an exact match in offset and size. (#81313)

Motivation for this and follow-on patches is to improve codegen for
libc++, where using memcpy limits optimizations, like vectorization for
code iteration over std::vector<std::complex<float>>:
https://godbolt.org/z/f3vqYos3c

Depends on https://github.com/llvm/llvm-project/pull/81289.

PR: https://github.com/llvm/llvm-project/pull/81313