tree-optimization/122569 - recognize CLZ via isolated MSB DeBruijn lookup

Recognize a CLZ idiom where the OR-cascade is followed by
(value - (value >> 1)) to isolate the MSB as a power of two (2^k), then
a DeBruijn multiply-and-shift maps 2^k back to k:

  value |= value >> 1;
  ...
  value |= value >> 32;
  result = table[((value - (value >> 1)) * MAGIC) >> 58];

After the cascade value is 2^(k+1) - 1, so (value - (value >> 1)) is 2^k
and the multiply-and-shift is a CTZ-style DeBruijn lookup whose table
satisfies table[(magic << k) >> shift] == k.

Add match.pd pattern clz_msb_iso_table_index on top of the
msb_or_cascade_64 helper, so it only spells out the (s - (s >> 1))
isolation and the DeBruijn shape.  simplify_count_zeroes validates the
table with the existing CTZ checkfn (the direct-form check) but emits
IFN_CLZ; both forms store MSB positions, so the CLZ path including
zero_val pre-compensation is unchanged.

Relax the element-type check from "precision <= 32" to "integral and
precision <= 64" so tables declared as unsigned long (64-bit on LP64)
are accepted; the values are bit positions and fit any integer type.

Only a 64-bit variant is added; all known uses (Stockfish, zstd,
cpython, the PR122569 comment 3 reproducer) are 64-bit.

gcc/ChangeLog:

	PR tree-optimization/122569
	* match.pd (clz_msb_iso_table_index): New match pattern.
	* tree-ssa-forwprop.cc (gimple_clz_msb_iso_table_index): Declare.
	(simplify_count_zeroes): Recognize the new pattern; route its
	table validation through the CTZ checkfn.  Relax the element
	type check to accept integer types up to 64 bits.

gcc/testsuite/ChangeLog:

	PR tree-optimization/122569
	* gcc.dg/tree-ssa/pr122569-3.c: New test.