simplify-rtx: Add missing line for previous commit (PR78583)

The comment for the added case to simplify_truncation reads

  /* Turn (truncate:M1 (*_extract:M2 (reg:M2) (len) (pos))) into
     (*_extract:M1 (truncate:M1 (reg:M2)) (len) (pos')) if possible without
     changing len.  */

but I forgot to check the two modes M2 are actually the same.


	PR rtl-optimization/78583
	* simplify-rtx.c (simplify_truncation): Add check missing from the
	previous commit.

From-SVN: r243000