Revert r185257 (InstCombine: Be more agressive optimizing 'udiv' instrs with 'select...

Revert r185257 (InstCombine: Be more agressive optimizing 'udiv' instrs with 'select' denoms)

I'm reverting this commit because:

 1. As discussed during review, it needs to be rewritten (to avoid creating and
then deleting instructions).

 2. This is causing optimizer crashes. Specifically, I'm seeing things like
this:

    While deleting: i1 %
    Use still stuck around after Def is destroyed:  <badref> = select i1 <badref>, i32 0, i32 1
    opt: /src/llvm-trunk/lib/IR/Value.cpp:79: virtual llvm::Value::~Value(): Assertion `use_empty() && "Uses remain when a value is destroyed!"' failed.

   I'd guess that these will go away once we're no longer creating/deleting
instructions here, but just in case, I'm adding a regression test.

Because the code is bring rewritten, I've just XFAIL'd the original regression test. Original commit message:

	InstCombine: Be more agressive optimizing 'udiv' instrs with 'select' denoms

	Real world code sometimes has the denominator of a 'udiv' be a
	'select'.  LLVM can handle such cases but only when the 'select'
	operands are symmetric in structure (both select operands are a constant
	power of two or a left shift, etc.).  This falls apart if we are dealt a
	'udiv' where the code is not symetric or if the select operands lead us
	to more select instructions.

	Instead, we should treat the LHS and each select operand as a distinct
	divide operation and try to optimize them independently.  If we can
	to simplify each operation, then we can replace the 'udiv' with, say, a
	'lshr' that has a new select with a bunch of new operands for the
	select.

llvm-svn: 185415