rs6000: clean up headers in rs6000.c and rs6000-call.c

rs6000: clean up headers in rs6000.c and rs6000-call.c

When Andrew Macleod investigated the recent rs6000 bootstrap failure,
he suggested a clean up of the headers in rs6000.c and rs6000-call.c.
It now is recommended to include ssa.h instead of the individual headers.
This also ensures that value-range.h is included and in the correct order
so that the tree-ssa-propagate.h inclusion of value-query.h and its
dependencies are satisfied.

Bootstrapped on powerpc-ibm-aix7.2.0.0 and powerpc64le-linux.

gcc/ChangeLog:

2020-10-02  David Edelsohn  <dje.gcc@gmail.com>
	    Andrew MacLeod  <amacleod@redhat.com>

	* config/rs6000/rs6000.c: Include ssa.h. Reorder some headers.
	* config/rs6000/rs6000-call.c: Same.