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

[Fei Yang]

On Tue, 30 Aug 2022 11:12:30 GMT, Fei Yang <fyang at openjdk.org> wrote:

> 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.

@shipilev : Want to take a look? Thanks.

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