The BLX instruction is encoded differently than the BL, because why not? In

The BLX instruction is encoded differently than the BL, because why not? In
particular, the immediate has 20-bits of value instead of 21. And bit 0 is '0'
always. Going through the BL fixup encoding was trashing the "bit 0 is '0'"
invariant.

Attempt to get the encoding at slightly more correct with this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121336 91177308-0d34-0410-b5e6-96231b3b80d8