swift234

Enhance IRGen of select_enum[_addr] to use the new 
emitValueCaseTest/emitIndirectCaseTest implementations when testing for
one case.  This leads to dramatically better IRGen for optional (among other
things).  For example, for:

class C{}
var x : C?
if let a = x {
}

we used to produce:

  %15 = load i64* %14, align 8
  switch i64 %15, label %16 [
    i64 0, label %17
  ]

; <label>:16                                      ; preds = %entry
  br label %select_enum

; <label>:17                                      ; preds = %entry
  br label %select_enum

select_enum:                                      ; preds = %17, %16
  %18 = phi i1 [ true, %16 ], [ false, %17 ]
  br i1 %18, label %22, label %19

now we produce:

  %15 = load i64* %14, align 8
  %16 = icmp eq i64 %15, 0
  %17 = xor i1 %16, true
  br i1 %17, label %21, label %18

Hopefully this makes a measurable improvement in the -O0 performance tests, but
I didn't check.

This resolves:
<rdar://problem/19404937> select_enum/select_enum_addr should generate select in obvious cases [-O0 performance]



Swift SVN r24366