Bug 344382

MSVC  2013 and possibly earlier generate optimised code for bitfield assignments
which confuses Memcheck.  This is at -O1 and -O2.  Doesn't happen at -Od.

The basic problem is:
to create a new word W, which consists of bits from A where a
mask M has a 0 bit and bits of B where M has a 1 bit, MSVC generates

   A ^ ((A ^ B) & M)

whereas gcc generates

  (A ^ ~M) | (B & M)

Memcheck has no problem with gcc's code, but MSVC's code causes a lot
of complaining.  This is because Memcheck doesn't have a way to know that
xoring an undefined value with itself produces a defined zero, since that
requires tracking the actual identity of the value, which Memcheck can't do.

Proposal is to transform the MSVC version into the GCC version in the
IR optimiser (ir_opt.c).  This seems to be pretty effective in initial trials.


Reproducible: Always