[tablegen] Avoid creating a temporary vector in getInstructionCase

[tablegen] Avoid creating a temporary vector in getInstructionCase

Record::getValues returns ArrayRef which has a cast operator
to std::vector, as a result a temporary vector is created
if the type of the variable is const std::vector&
that is suboptimal in this case.

Differential revision: https://reviews.llvm.org/D34969

Test plan: make check-all

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307063 91177308-0d34-0410-b5e6-96231b3b80d8