RFR: 8278037: Removed PPC32 dead code in opConvert

[Ao Qi]

`_tmp1` and `_tmp2` were [removed](https://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/faaed259df37#l13.173) in [JDK-8160245](https://bugs.openjdk.java.net/browse/JDK-8160245), but they are still used.

This should cause a build error. I don't have a ppc32 machine for the test. It's also not found at https://builds.shipilev.net. Is ppc32 a supported compiler1 platform?

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Commit messages:
 - 8278037: Removed PPC32 dead code in opConvert

Changes: https://git.openjdk.java.net/jdk/pull/6625/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6625&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278037
  Stats: 10 lines in 1 file changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6625.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6625/head:pull/6625

PR: https://git.openjdk.java.net/jdk/pull/6625