Set correct <def,undef> flags when lowering REG_SEQUENCE.

A REG_SEQUENCE instruction is lowered into a sequence of partial defs:

  %vreg7:ssub_0<def,undef> = COPY %vreg20:ssub_0
  %vreg7:ssub_1<def> = COPY %vreg2
  %vreg7:ssub_2<def> = COPY %vreg2
  %vreg7:ssub_3<def> = COPY %vreg2

The first def needs an <undef> flag to indicate it is the beginning of
the live range, while the other defs are read-modify-write.  Previously,
we depended on LiveIntervalAnalysis to notice and fix the missing
<def,undef>, but that solution was never robust, it was causing problems
with ProcessImplicitDefs and the lowering of chained REG_SEQUENCE
instructions.

This fixes PR11841.

llvm-svn: 148879