[ArgumentPromotion] Fix byval alignment handling.

[ArgumentPromotion] Fix byval alignment handling.

Make sure the alignment of the generated operations matches the
alignment of the byval argument.  Previously, we were just ignoring
alignment and getting lucky.

While I'm here, also delete the unnecessary "tail" handling.
Passing a pointer to a byval argument to a "tail" call is UB, so
rewriting to an alloca doesn't require any special handling.

Differential Revision: https://reviews.llvm.org/D89819