[InstCombine] Remove unneeded internal function 'decomposeSimpleLinearExpr' in...

[InstCombine] Remove unneeded internal function 'decomposeSimpleLinearExpr' in InstCombineCasts.cpp (NFC)

/data/llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:32:15: error: function 'decomposeSimpleLinearExpr' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static Value *decomposeSimpleLinearExpr(Value *Val, unsigned &Scale,
              ^
1 error generated.