Merging r308808, r308813 and r308906:

Merging r308808, r308813 and r308906:
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r308808 | arsenm | 2017-07-21 16:56:13 -0700 (Fri, 21 Jul 2017) | 6 lines

RA: Remove assert on empty live intervals

This is possible if there is an undef use when
splitting the vreg during spilling.

Fixes bug 33620.
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r308813 | arsenm | 2017-07-21 17:24:01 -0700 (Fri, 21 Jul 2017) | 6 lines

RA: Remove another assert on empty intervals

This case is similar to the one fixed in r308808,
except when rematerializing.

Fixes bug 33884.
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r308906 | arsenm | 2017-07-24 11:07:55 -0700 (Mon, 24 Jul 2017) | 6 lines

RA: Replace asserts related to empty live intervals

These don't exactly assert the same thing anymore, and
allow empty live intervals with non-empty uses.

Removed in r308808 and r308813.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@309171 91177308-0d34-0410-b5e6-96231b3b80d8