RFR: 8310735: Build failure after JDK-8310577 with GCC8

[Jie Fu]

Since `enum BasicType` had been changed to `enum BasicType : u1`, `basic_type >= 0` would be always true.
So it's safe to remove such conditions in these asserts.
Thanks.

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Commit messages:
 - 8310735: Build failure after JDK-8310577 with GCC8

Changes: https://git.openjdk.org/jdk/pull/14627/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14627&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310735
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14627.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14627/head:pull/14627

PR: https://git.openjdk.org/jdk/pull/14627