SelectionDAG switch lowering: Replace unreachable default with most popular case.

This can significantly reduce the size of the switch, allowing for more
efficient lowering.

I also worked with the idea of exploiting unreachable defaults by
omitting the range check for jump tables, but always ended up with a
non-neglible binary size increase. It might be worth looking into some more.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223049 91177308-0d34-0410-b5e6-96231b3b80d8
SimplifyCFG: don't remove unreachable default switch destinations

An unreachable default destination can be exploited by other optimizations, and
SDag lowering is now prepared to handle them efficiently.

For example, branches to the unreachable destination will be optimized away,
such as in the case of range checks for switch lookup tables.

On 64-bit Linux, this reduces the size of a clang bootstrap by 80 kB (and
Chromium by 30 kB).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223050 91177308-0d34-0410-b5e6-96231b3b80d8
SimplifyCFG: Omit range checks for switch lookup tables when default is unreachable

They would get optimized away later, but we might as well not emit them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223051 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r223049, r223050 and r223051 while investigating test failures.

I didn't foresee affecting the Clang test suite :/

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223054 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: lower tail calls correctly when using GHC calling convention.

Patch by Ben Gamari.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223055 91177308-0d34-0410-b5e6-96231b3b80d8