Allow more complex call replacements in gimple-fold.c

Allow more complex call replacements in gimple-fold.c

An upcoming patch adds a match.pd rule that folds pow(pow(x,y),z)
to pow(x,y*z).  This fold can reuse the existing pow gimple statement
and simply replace the operands with x and y*z.  However, the y*z
itself requires a separate gimple statement and the code wasn't
prepared to handle that.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* gimple-fold.c (replace_stmt_with_simplification): Don't allow
	new statements to be inserted if inplace.  Allow calls to have
	nonempty sequences.

From-SVN: r229371