[Power9] Implement new altivec instructions: bcd* series

This patch implements the following altivec instructions:

- Decimal Convert From/to National/Zoned/Signed-QWord:
    bcdcfn. bcdcfz. bcdctn. bcdctz. bcdcfsq. bcdctsq.

- Decimal Copy-Sign/Set-Sign:
    bcdcpsgn. bcdsetsgn.

- Decimal Shift/Unsigned-Shift/Shift-and-Round:
    bcds. bcdus. bcdsr.

- Decimal (Unsigned) Truncate:
    bcdtrunc. bcdutrunc.

Total 13 instructions

Thanks Amehsan's advice! Thanks Kit's great help!
Reviewers: hal, nemanja, kbarton, tjablin, amehsan

http://reviews.llvm.org/D17838

llvm-svn: 264568