cselim: Move else_vdef definition to the usage

cselim: Move else_vdef definition to the usage

This is based on the review of cond_if_else_store_replacement_limited
(https://gcc.gnu.org/pipermail/gcc-patches/2025-June/686076.html).

Moves else_vdef definition/assignment to be next to the usage in
single_trailing_store_in_bb.

Pushed as obvious after test on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (cond_if_else_store_replacement): Move
	definitin of else_vdef to right before the usage. Reformat
	slightly.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>