[X86] Make isel select the shorter form of jump instructions instead of the long...

[X86] Make isel select the shorter form of jump instructions instead of the long form.

The assembler backend will relax to the long form if necessary. This removes a swap from long form to short form in the MCInstLowering code. Selecting the long form used to be required by the old JIT.

llvm-svn: 225242