Check for matching CONST_VECTOR encodings [PR99929]

Check for matching CONST_VECTOR encodings [PR99929]

PR99929 is one of those “how did we get away with this for so long”
bugs: the equality routines weren't checking whether two variable-length
CONST_VECTORs had the same encoding.  This meant that:

   { 1, 0, 0, 0, 0, 0, ... }

would appear to be equal to:

   { 1, 0, 1, 0, 1, 0, ... }

since both are represented using the elements { 1, 0 }.

gcc/
	PR rtl-optimization/99929
	* rtl.h (same_vector_encodings_p): New function.
	* cse.c (exp_equiv_p): Check that CONST_VECTORs have the same encoding.
	* cselib.c (rtx_equal_for_cselib_1): Likewise.
	* jump.c (rtx_renumbered_equal_p): Likewise.
	* lra-constraints.c (operands_match_p): Likewise.
	* reload.c (operands_match_p): Likewise.
	* rtl.c (rtx_equal_p_cb, rtx_equal_p): Likewise.

(cherry picked from commit a87d3f964df31d4fbceb822c6d293e85c117d992)