rs6000: Disable mode promotion for pseudos

rs6000: Disable mode promotion for pseudos

rs6000 has instructions that can do almost everything 32 bit
at least as efficiently as corresponding 64 bit things. The
mode promotion can be defered to when a wide mode is necessary.
So it helps a lot not promote mode for pseudos. SPECint test
shows that the overall performance improvement (by geomean) is
more than 2% with this patch.

testsuite/gcc.target/powerpc/not-promote-mode.c illustrates how
the patch eliminates the redundant extensions and do further
optimization by disabling mode promotion for pseduos.

gcc/ChangeLog

	* config/rs6000/rs6000.h (PROMOTE_MODE): Remove.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/not-promote-mode.c: New.

(cherry picked from commit 9080a3bf23297885fdc47221da37a71d6dec93c5)