[test/Transforms/InstCombine/shift.ll] Removed duplicate test cases. NFC.

Removed some duplicate test cases from the file /test/Transforms/InstCombine/shift.ll.

test54 and test57 were duplicates of each other.
test55 and test58 were duplicates of each other.

(Removed test57 and test58)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223767 91177308-0d34-0410-b5e6-96231b3b80d8