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

[Aleksey Shipilev]

Happens now in master:


$ CONF=linux-x86-server-fastdebug make run-test TEST=compiler/loopopts/superword/CoLocatePack.java TEST_VM_OPTS="-XX:UseAVX=0 -XX:UseSSE=0"
...

CompileCommand: compileonly compiler/loopopts/superword/CoLocatePack.test bool compileonly = true
 191 ConvF2L === _ 714 [[ 193 ]] !jvms: CoLocatePack::test @ bci:30 (line 70)
# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/phaseX.cpp:1128
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/shade/trunks/jdk/src/hotspot/share/opto/phaseX.cpp:1128), pid=1717516, tid=1717532
# fatal error: modified node was not processed by IGVN.transform_old()


After JDK-8266950 (always `strictfp`), the paths in `Conv(D|F)2(I|L)Nodes::Ideal`-s start to be taken more frequently to round float/double inputs when low SSE is enabled. On those paths, we call `set_req` to rewire current node, but we still return `NULL` from `::Ideal`. I believe that is incorrect, as per `node.cpp` explanation: `NULL` indicates no graph change was done, and `this` should be returned when modification happened. So GVN predictably barfs.

Additional testing:
 - [x] Linux x86_32 `tier1` with `-XX:UseAVX=0 -XX:UseSSE=0` (now pass)
 - [x] Linux x86_32 `tier1` default (still pass)
 - [ ] Linux x86_64 `tier1` default

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

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/6176/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6176&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/jdk/pull/6176.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6176/head:pull/6176

PR: https://git.openjdk.java.net/jdk/pull/6176