swift234

Teach CFGBuilder a simple peephole for tupleelement(tuple()) to just return
the element of the tuple.  This is a common case because CFGGen shouldn't
(and doesn't) try to worry about this sort of thing.  Before we'd get
CFGIR like this:

  %15 = tuple (%4, %9, %14)
  %16 = alloc_array element type=Int, 3
  %17 = tupleelement %16, 1
  %18 = tupleelement %15, 0
  %19 = store %18 -> %17 [initialization]
  %20 = index_lvalue %17, 1
  %21 = tupleelement %15, 1
  %22 = store %21 -> %20 [initialization]
  %23 = index_lvalue %17, 2
  %24 = tupleelement %15, 2
  %25 = store %24 -> %23 [initialization]

now we get a dead tuple (%15), but much cleaner IR:

  %15 = tuple (%4, %9, %14)
  %16 = alloc_array element type=Int, 3
  %17 = tupleelement %16, 1
  %18 = store %4 -> %17 [initialization]
  %19 = index_lvalue %17, 1
  %20 = store %9 -> %19 [initialization]
  %21 = index_lvalue %17, 2
  %22 = store %14 -> %21 [initialization]



Swift SVN r2994