RegisterCoalescer: Gracefully continue if subrange merging fails.
RegisterCoalescer: Gracefully continue if subrange merging fails.

There is a known bug where the register coalescer fails to merge
subranges when multiple ranges end up in the "overflow" bit 32 of the
lanemasks. A proper fix for this is complicated so for now this is a
workaround which lets the register coalescer drop the subregister
liveness information (we just loose some precision by that) and
continue.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@231186 91177308-0d34-0410-b5e6-96231b3b80d8

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