Simplify cst_and_fits_in_hwi

Simplify cst_and_fits_in_hwi

While looking at the use of cst_and_fits_in_hwi in tree-ssa-loop-ivopts.c,
I had difficulty working out what the function actually tests.  The
final NUNITS check seems redundant, since it asks about the number of
HWIs in the _unextended_ constant.  We've already checked that the
unextended constant has no more than HOST_BITS_PER_WIDE_INT bits, so the
length must be 1.

I think this was my fault, sorry.

Tested on x86_64-linux-gnu and aarch64-linux-gnu.

gcc/
	* tree.c (cst_and_fits_in_hwi): Simplify.

From-SVN: r235722