[PATCH] [AArch64] Add v8.1a "Rounding Doubling Multiply Add/Subtract" extension

[Vladimir Sukharev]

1. In AArch64ISelLowering.cpp , function ReplaceNodeResults() , we need also "sqsub"
2. I don't think i32 sqrdmlah() will work right, even if we'd replace ISD::TRUNCATE with SQXTN. Is the following correct?

i16 sqrdmlah (0, 100, 1000) -> 0 sqadd (100 sqrdmulh 1000) -> 0 sqadd (high i16 half of 100000) ->  0 sqadd 6 -> 6  - we need to obtain that with workaround...
i32 sqrdmlah (0, 1000, 1000) -> 0 sqadd (high i32 half of 100000) -> 0
nope, workaround does not seem to be right


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http://reviews.llvm.org/D8502

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