Reverted part of r59335:...

Reverted part of r59335: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20081110/009243.html
In that patch I added a bogus type promotion for unary '!'.

The real bug was more fallout from edges cases with compound assignments and conjured symbolic values.  Now the conjured value has the type of the LHS expression, and we do a promotion to the computation type.  We also now correctly do a conversion from the computation type back to the LHS type.

llvm-svn: 59349