Teach liveintervalanalysis about virtual registers which are defined by reg_sequence...

Teach liveintervalanalysis about virtual registers which are defined by reg_sequence instructions that are formed by registers defined by distinct instructions. e.g.
80      %reg1041:6<def> = VSHRNv4i16 %reg1034<kill>, 12, pred:14, pred:%reg0
. . .
120     %reg1041:5<def> = VSHRNv4i16 %reg1039<kill>, 12, pred:14, pred:%reg0

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103102 91177308-0d34-0410-b5e6-96231b3b80d8