Smaller than word size integers must be promoted to word size
when passed on the stack. While on little endian systems we can
get away with writing a small integer to a word size stack slot
and read it as a word ignoring the upper bits, on big endian
systems a small integer write ends up in the most significant
bits and a word size read that ignores the upper bits delivers
a random value.

On little endian systems a smaller than word size write to
the stack might be more efficient but that decision is
system specific and should be done as an optimization in the
respective backends.

Fixes #16258

(cherry picked from commit af7b0fdb)