fortran: Pre-evaluate string pointers. [PR102043]

fortran: Pre-evaluate string pointers. [PR102043]

This avoids a regression on deferred_character_23.f90 later in the
patch series when array references are rewritten to use pointer
arithmetic.

The problem is a SAVE_EXPR tree as TYPE_SIZE_UNIT of one array element
type, which is used by the pointer arithmetic expressions.  As these
expressions appear in both branches of an if-then-else block, the tree
is lowered to a variable in one of the branches but it’s used in both
branches, which is invalid middle-end code.

This change pre-evaluates the array references or pointer arithmetics
to variables before the if-then-else block, so that the SAVE_EXPR are
expanded to variables in the parent scope of the if-then-else block,
and expressions referencing the variables remain valid in both
branches.

	PR fortran/102043

gcc/fortran/ChangeLog:
	* trans-expr.cc: Pre-evaluate src and dest to variables
	before using them.

gcc/testsuite/ChangeLog:
	* gfortran.dg/dependency_49.f90: Update variable occurence
	count.