optimize & rmv from public API Perl_tmps_grow and related code

optimize & rmv from public API Perl_tmps_grow and related code

Previously in PUSH_EXTEND_MORTAL__SV_C, "PL_tmps_ix + 1" would execute
twice, once for the nonmutable if(>=), then again after the potential
tmps_grow call. tmps_grow has an unused return register/void proto, put it
to use by returning ix.  Also change tmps_grow to take the result of
"PL_tmps_ix + the constant (usually 1) or non-constant (EXTEND_MORTAL)".
This avoid having to put the constant twice in machine code, once for the
if test, 2nd time for extend length param for tmps_grow call. For
non-constant/EXTEND_MORTAL usage, it allows the C optimizer to have the
length var to go out of liveness sooner if possible. Also the var used for
the if(>=) test is more likely to be in a register than length var.
So "if test variable" is closer on hand to the CPU than length var. In some
cases, if non-const len var isn't used again, it becomes the "ix" variable
by having PL_tmps_ix added to it. Change sv_2mortal to return sv instead
of NULL to remove a unique branch/block of machine code that assigns 0 to
return variable (Visual C didn't figure out return sv == returned NULL,
not sv). See also [perl #121845].