[x86] try harder to match bitwise 'or' into an LEA

The motivation for this patch starts with the epic fail example in PR18007:
https://llvm.org/bugs/show_bug.cgi?id=18007

...unfortunately, this patch makes no difference for that case, but it solves some
simpler cases. We'll get there some day. :)

The current 'or' matching code was using computeKnownBits() via 
isBaseWithConstantOffset() -> MaskedValueIsZero(), but that's an unnecessarily limited use. 
We can do more by copying the logic in ValueTracking's haveNoCommonBitsSet(), so we can 
treat the 'or' as if it was an 'add'.

There's a TODO comment here because we should lift the bit-checking logic into a helper
function, so it's not duplicated in DAGCombiner.

An example of the better LEA matching:

leal (%rdi,%rdi), %eax
andl $1, %esi
orl %esi, %eax

Becomes:

andl $1, %esi
leal (%rsi,%rdi,2), %eax

Differential Revision: http://reviews.llvm.org/D13956

llvm-svn: 252515