perlop: Mention ~ among ops that vary by type
op.h: Parenthesize macro args for cUNOPx etc.

Without this, we cannot do cUNOPx(complex expression) without worrying
about precedence issues.
op.c: Remove assert exception for OP_REPEAT

I changed the op tree in v5.21.5-101-g5e46266.
pp_sys.c:pp_gmtime: Remove redundant EXTEND_MORTAL

The purpose of EXTEND_MORTAL is to extend the mortals stack only
once before pushing multiple items.  Here we are just going to push
one item, so extending it gains us nothing, as sv_2mortal is going
to do that anyway.

Moreover, we will not even be using the mortals stack if there has
been an error.