Merge, from CGTUNE branch, a cleaned up version of r6742:

Merge, from CGTUNE branch, a cleaned up version of r6742:

Another optimisation: allow tools to provide a final_tidy function
which they can use to mess with the final post-tree-built IR before it
is handed off to instruction selection.

In memcheck, use this to remove redundant calls to
MC_(helperc_value_check0_fail) et al.  Gives a 6% reduction in code
size for Memcheck on x86 and a smaller (3% ?) speedup.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6787