Switch-to-lookup tables: set threshold to 3 cases

There has been an old FIXME to find the right cut-off for when it's worth
analyzing and potentially transforming a switch to a lookup table.

The switches always have two or more cases. I could not measure any speed-up
by transforming a switch with two cases. A switch with three cases gets a nice
speed-up, and I couldn't measure any compile-time regression, so I think this
is the right threshold.

In a Clang self-host, this causes 480 new switches to be transformed,
and reduces the final binary size with 8 KB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199294 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify x86 disassembler table handling of when to use TYPE_Rv/TYPE_R16/TYPE_R32 now that HasOpSizePrefix only means 16-bit instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199295 91177308-0d34-0410-b5e6-96231b3b80d8
For AArch64, lowering sext_inreg and generate optimized code by using SXTL.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199296 91177308-0d34-0410-b5e6-96231b3b80d8
Add OpSize16 to the two byte forms of INC/DEC that we only use in 64-bit mode and a 64-bit only LEA. Even though we'll not be in 16-bit mode when we use them it makes their tables consistent with their 32-bit counterparts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199297 91177308-0d34-0410-b5e6-96231b3b80d8