Henrik Gramner authored Mar 13, 2024 and Henrik Gramner committed Mar 14, 2024

Prior to this change dealing with the scenario where the number of
XMM registers spilled depends on if a branch is taken or not was
complicated to handle well. There was essentially three options:

1) Always spill the largest number of XMM register. Results in
   unnecessary spills.

2) Do the spilling after the branch. Results in code duplication
   for the shared subset of spills.

3) Do the spilling manually. Optimal, but overly complex and vexing.

This adds an additional optional argument to the WIN64_SPILL_XMM
and WIN64_PUSH_XMM macros to make it possible to allocate space
for a certain number of registers but initially only push a subset
of those, with the option of pushing additional register later.