[x86] fix usage of stale operands when lowering select

I noticed this problem as part of the ongoing attempt to canonicalize min/max ops in IR.

The debug output shows nodes like this:

t4: i32 = xor t2, Constant:i32<-1>
    t21: i8 = setcc t4, Constant:i32<0>, setlt:ch
  t14: i32 = select t21, t4, Constant:i32<-1>

And because the select is holding onto the t4 (xor) node while EmitTest creates a new 
x86-specific xor node, the lowering results in:

  t4: i32 = xor t2, Constant:i32<-1>
  t25: i32,i32 = X86ISD::XOR t2, Constant:i32<-1>
t28: i32,glue = X86ISD::CMOV Constant:i32<-1>, t4, Constant:i8<15>, t25:1

Differential Revision: https://reviews.llvm.org/D28374

llvm-svn: 291392