Implement a bunch of symbolic constant folding opportunities. This implements

testcase test/Regression/Assembler/ConstantExprFold.llx

Note that these kinds of things only rarely show up in source code, but are
exceedingly common in the intermediate stages of algorithms like SCCP.  By
folding things (especially relational operators) that use symbolic constants,
we are able to speculatively fold more conditional branches, which can
lead to some big simplifications.

It would be easy to add a lot more special cases here, so if you notice
SCCP missing anything "obvious", you know what to make smarter.  :)

llvm-svn: 10812