ir3/ra: Make sure we don't pick a preferred reg overflowing the file.

ir3/ra: Make sure we don't pick a preferred reg overflowing the file.

If we're in handle_collect()'s dst allocation and are part of a merge set
near the end of the file, our check for reg_elem_size(reg) would let us
use the preferred reg when that would immediately lead to
allocate_dst_fixed() creating an interval extending thruogh reg_size(reg)
that overflows the file.

Avoids a regression on gfxbench5/gl_5_high_off/17.shader_test in the next
commit.  No change on shader-db.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18946>