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[mips] Merge MipsLongBranch and MipsHazardSchedule passes

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[mips] Merge MipsLongBranch and MipsHazardSchedule passes

MipsLongBranchPass and MipsHazardSchedule passes are joined to one pass
because of mutual conflict. When MipsHazardSchedule inserts 'nop's, it
potentially breaks some jumps, so they have to be expanded to long
branches. When some branch is expanded to long branch, it potentially
creates a hazard situation, which should be fixed by adding nops.
New pass is called MipsBranchExpansion, it combines these two passes,
and runs them alternately until one of them reports no changes were made.

Differential Revision: https://reviews.llvm.org/D46641

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