Merging r339260:

------------------------------------------------------------------------
r339260 | syzaara | 2018-08-08 08:20:43 -0700 (Wed, 08 Aug 2018) | 13 lines

[PowerPC] Improve codegen for vector loads using scalar_to_vector

This patch aims to improve the codegen for vector loads involving the
scalar_to_vector (load X) sequence. Initially, ld->mv instructions were used
for scalar_to_vector (load X), so this patch allows scalar_to_vector (load X)
to utilize:

LXSD and LXSDX for i64 and f64
LXSIWAX for i32 (sign extension to i64)
LXSIWZX for i32 and f64

Committing on behalf of Amy Kwan.
Differential Revision: https://reviews.llvm.org/D48950
------------------------------------------------------------------------

llvm-svn: 347957