fold-const: Fix ICE in fold_read_from_constant_string on invalid code [PR99204]

fold_read_from_constant_string and expand_expr_real_1 have code to optimize
constant reads from string (tree vs. rtl).
If the STRING_CST array type has zero low bound, index is fold converted to
sizetype and so the compare_tree_int works fine, but if it has some other
low bound, it calls size_diffop_loc and that function from 2 sizetype
operands creates a ssizetype difference.  expand_expr_real_1 then uses
tree_fits_uhwi_p + compare_tree_int and so works fine, but fold-const.c
only checked if index is INTEGER_CST and calls compare_tree_int, which means
for negative index it will succeed and result in UB in the compiler.

2021-02-23  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/99204
	* fold-const.c (fold_read_from_constant_string): Check that
	tree_fits_uhwi_p (index) rather than just that index is INTEGER_CST.

	* gfortran.dg/pr99204.f90: New test.

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