[TTI] BasicTTIImpl assumes no vector registers

Summary:
Following the discussion on r241884, it's more reasonable to assume that a
target has no vector registers by default instead of letting every such
target overrides getNumberOfRegisters.

Therefore, this patch modifies BasicTTIImpl::getNumberOfRegisters to
return 0 when Vector is true, and partially reverts r241884 which
modifies NVPTXTTIImpl::getNumberOfRegisters.

It also fixes a performance bug in LoopVectorizer. Even if a target has
no vector registers, vectorization may still help ILP. So, we need both
checks to be false before disabling loop vectorization all together.

Reviewers: hfinkel

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D11108

llvm-svn: 241942