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

Add the missing piece of r372029.
Somehow when the patch for review D61961 was committed, only the test case
went in and the code didn't. This of course caused all kinds of build bot
breaks.
This patch just adds the code for that patch.

Author: Lei Huang
Differential revision: https://reviews.llvm.org/D61961

llvm-svn: 372043