LoopVectorize: We don't need an identity element for min/max reductions

We can just use the initial element that feeds the reduction.

  max(max(x, y), z) == max(max(x,y), max(x,z))

radar://13723044
LoopVectorizer: Cleanup of miminimum/maximum pattern match code

No need for setting the operands. The pointers are going to be bound by the
matcher.

radar://13723044
PatternMatch: Matcher for (un)ordered floating point min/max

Add support for matching 'ordered' and 'unordered' floating point min/max
constructs.

In LLVM we can express min/max functions as a combination of compare and select.
We have support for matching such constructs for integers but not for floating
point. In floating point math there is no total order because of the presence of
'NaN'. Therefore, we have to be careful to preserve the original fcmp semantics
when interpreting floating point compare select combinations as a minimum or
maximum function. The resulting 'ordered/unordered' floating point maximum
function has to select the same value as the select/fcmp combination it is based
on.

 ordered_max(x,y)   = max(x,y) iff x and y are not NaN, y otherwise
 unordered_max(x,y) = max(x,y) iff x and y are not NaN, x otherwise
 ordered_min(x,y)   = min(x,y) iff x and y are not NaN, y otherwise
 unordered_min(x,y) = min(x,y) iff x and y are not NaN, x otherwise

This matches the behavior of the underlying select(fcmp(olt/ult/.., L, R), L, R)
construct.

Any code using this predicate has to preserve this semantics.

A follow-up patch will use this to implement floating point min/max reductions
in the vectorizer.

radar://13723044
LoopVectorize: Add support for floating point min/max reductions

Add support for min/max reductions when "no-nans-float-math" is enabled. This
allows us to assume we have ordered floating point math and treat ordered and
unordered predicates equally.

radar://13723044