Move early tail duplication earlier.

Move early tail duplication earlier.

This fixes the issue noted in PR10251 where early tail dup of bbs with
indirectbr would cause a bb to be duplicated into a loop preheader
and then into its predecessors, creating phi nodes with identical
operands just before register allocation.

This helps with jsinterp.o size (__TEXT goes from 163568 to 126656)
and a bit with performance 1.005x faster on sunspider (jits still enabled).

The result on webkit with the jit disabled is more significant: 1.021x faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134372 91177308-0d34-0410-b5e6-96231b3b80d8