[Legalizer] Only allocate the SetVectors once per function.

[Legalizer] Only allocate the SetVectors once per function.

Prior to this patch we used to create SetVectors in temporaries that
were created and destroyed for each instruction. Now, instead we create
and destroyed them only once, but clear the content for each
instruction.
This speeds up the pass by ~25%.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315756 91177308-0d34-0410-b5e6-96231b3b80d8