[NFC][CodeGen][X86][AArch64] Add tests for C++ std::midpoint() pattern (PR40965)

Tests only for integers, not floating point or pointers.

The scalar 8-bit case uses branch instead of CMOV,
because there is no no 8-bit CMOV.

Vector tests are for consistency, since it can be vectorized.

https://bugs.llvm.org/show_bug.cgi?id=40965

llvm-svn: 355436