Fix PR78588 - rtlanal.c:5210:38: runtime error: shift exponent 4294967295 is too large for 64-bit type

Building gcc with -fsanitize=undefined shows:
 rtlanal.c:5210:38: runtime error: shift exponent 4294967295 is too large for 64-bit type 'long unsigned int'

This happens because if_then_else_cond() in combine.c calls
num_sign_bit_copies() in rtlanal.c with mode==BLKmode.

5205   bitwidth = GET_MODE_PRECISION (mode);
5206   if (bitwidth > HOST_BITS_PER_WIDE_INT)
5207     return 1;
5208
5209   nonzero = nonzero_bits (x, mode);
5210   return nonzero & (HOST_WIDE_INT_1U << (bitwidth - 1))
5211          ? 1 : bitwidth - floor_log2 (nonzero) - 1;

This causes (bitwidth - 1) to wrap around.

	PR rtl-optimization/78588
	* combine.c (if_then_else_cond): Also guard against BLKmode.
	* rtlanal.c (num_sign_bit_copies1): Add assert.

From-SVN: r242997