[VE] Change threshold for jump table generation

Implement getMinimumJumpTableEntries() to specify threshold for jump
table genaration.  We use 8 for the case of PIC mode to relieve the
impact of PIC calculation required to implement PIC mode jump table.
Update jump table regression test also.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D91785