Add alternative support for FP_ROUND from v2f32 to v2f64

- Due to the current matching vector elements constraints in ISD::FP_EXTEND,
  rounding from v2f32 to v2f64 is scalarized. Add a customized v2f32 widening
  to convert it into a target-specific X86ISD::VFPEXT to work around this
  constraints. This patch also reverts a previous attempt to fix this issue by
  recovering the scalarized ISD::FP_EXTEND pattern and thus significantly
  reduces the overhead of supporting non-power-2 vector FP extend.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165625 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed