Change pre-regalloc tail duplication to only duplicate indirect branch blocks.

Change pre-regalloc tail duplication to only duplicate indirect branch blocks.
The pre-regalloc pass caused some regressions in both compile time and
performance of the generated code, and it did not improve performance, except
for indirect branches.  I also moved the check for single-block loops to speed
up the common case when running the taildup pass before reg allocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93505 91177308-0d34-0410-b5e6-96231b3b80d8