False Positive for Performance/FixedSize #54
Description
Performance/FixedSize should not be triggered for constants definitions.
Expected behavior
SOME_CONST = "abc".length # this works ok
SOME_CONST = "abc".length + 1 # should also work and not trigger cop
Actual behavior
SOME_CONST = "abc".length # this works ok
SOME_CONST = "abc".length + 1 # this triggers Performance/FixedSize
Steps to reproduce the problem
SOME_CONST = "abc".length
SOME_CONST = "abc".length + 1
RuboCop version
0.67.2 (using Parser 2.6.2.1, running on ruby 2.5.3 x86_64-darwin18)