Merge, from CGTUNE branch:

Merge, from CGTUNE branch:

r1765:
During register allocation, keep track of which (real) registers have
the same value as their associated spill slot.  Then, if a register
needs to be freed up for some reason, and that register has the same
value as its spill slot, there is no need to produce a spill store.
This substantially reduces the number of spill store instructions
created.  Overall gives a 1.9% generated code size reduction for
perf/bz2 running on x86.

r1766:
Followup to r1765: fix some comments, and rearrange fields in struct
RRegState so as to fit it into 16 bytes.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1778