[PPC64LE] Remove implicit-subreg restriction from VSX swap removal

[PPC64LE] Remove implicit-subreg restriction from VSX swap removal

In r241285, I removed the SUBREG_TO_REG restriction from VSX swap
removal, determining that this was overly conservative.  We have
another form of the same restriction in that we check for the presence
of implicit subregs in vector operations.  As with SUBREG_TO_REG for
partial register conversions, an implicit subreg is safe in and of
itself, provided no other operation makes a lane-sensitive assumption
about the result.  This patch removes that restriction, by removing
the HasImplicitSubreg flag and all code that relies on it.

I've added a test case that fails to optimize before this patch is
applied, and optimizes properly with the patch.  Test based on a
report from Anton Blanchard.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241290 91177308-0d34-0410-b5e6-96231b3b80d8