RFR: 8293050: RISC-V: Remove redundant non-null assertions about macro-assembler

[Fei Yang]

There are some non-null assertions about macro-assembler in riscv-specific code. Most of them are there to make some internal code-check tools happy. We should remove those redundant assertions to make the code clean.

Tested with tier1 hotspot on linux-riscv64 unmatched.

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Commit messages:
 - 8293050: RISC-V: Remove redundant non-null assertions about macro-assembler

Changes: https://git.openjdk.org/jdk/pull/10079/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10079&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293050
  Stats: 54 lines in 9 files changed: 0 ins; 47 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/10079.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10079/head:pull/10079

PR: https://git.openjdk.org/jdk/pull/10079