[PowerPC] Cust lower fpext v2f32 to v2f64 from extract_subvector v4f32

[PowerPC] Cust lower fpext v2f32 to v2f64 from extract_subvector v4f32

This is a follow up patch from https://reviews.llvm.org/D57857 to handle
extract_subvector v4f32.  For cases where we fpext of v2f32 to v2f64 from
extract_subvector we currently generate on P9 the following:

  lxv 0, 0(3)
  xxsldwi 1, 0, 0, 1
  xscvspdpn 2, 0
  xxsldwi 3, 0, 0, 3
  xxswapd 0, 0
  xscvspdpn 1, 1
  xscvspdpn 3, 3
  xscvspdpn 0, 0
  xxmrghd 0, 0, 3
  xxmrghd 1, 2, 1
  stxv 0, 0(4)
  stxv 1, 0(5)

This patch custom lower it to the following sequence:

  lxv 0, 0(3)       # load the v4f32 <w0, w1, w2, w3>
  xxmrghw 2, 0, 0   # Produce the following vector <w0, w0, w1, w1>
  xxmrglw 3, 0, 0   # Produce the following vector <w2, w2, w3, w3>
  xvcvspdp 2, 2     # FP-extend to <d0, d1>
  xvcvspdp 3, 3     # FP-extend to <d2, d3>
  stxv 2, 0(5)      # Store <d0, d1> (%vecinit11)
  stxv 3, 0(4)      # Store <d2, d3> (%vecinit4)

Differential Revision: https://reviews.llvm.org/D61961

llvm-svn: 372029