[PowerPC] custom lower `v2f64 fpext v2f32`

Reduces scalarization overhead via custom lowering of v2f64 fpext v2f32.

eg. For the following IR
  %0 = load <2 x float>, <2 x float>* %Ptr, align 8
  %1 = fpext <2 x float> %0 to <2 x double>
  ret <2 x double> %1

Pre custom lowering:
  ld r3, 0(r3)
  mtvsrd f0, r3
  xxswapd vs34, vs0
  xscvspdpn f0, vs0
  xxsldwi vs1, vs34, vs34, 3
  xscvspdpn f1, vs1
  xxmrghd vs34, vs0, vs1

After custom lowering:
  lfd f0, 0(r3)
  xxmrghw vs0, vs0, vs0
  xvcvspdp vs34, vs0

Differential Revision: https://reviews.llvm.org/D57857

llvm-svn: 360429