[jdk17u] RFR: 8276105: C2: Conv(D|F)2(I|L)Nodes::Ideal should handle rounding correctly

[Aleksey Shipilev]

Clean backport to improve C2 x86_32 reliability.

Additional testing:
 - [x] Linux x86_32 fastdebug `tier1` passes

-------------

Commit messages:
 - Backport 0488ebdf14dacfa79d98de16ed949c813dd88701

Changes: https://git.openjdk.java.net/jdk17u/pull/261/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=261&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276105
  Stats: 16 lines in 1 file changed: 8 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/261.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/261/head:pull/261

PR: https://git.openjdk.java.net/jdk17u/pull/261