RFR: 8299960: Speed up test/hotspot/jtreg/compiler/c2/irTests/TestVectorizeURShiftSubword.java [v2]

[Emanuel Peter]

> This is one of the most time-consuming compiler tests.
> On my machine, I was able to reduce it from `20 sec` to `3 sec`.
> 
> This is an irTest, so we basically only want to see if certain IR-nodes are present.
> By default we do lots of warmup iterations, this is not required in these examples, so I set `@Warmup(1)`.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  Review suggestion

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11942/files
  - new: https://git.openjdk.org/jdk/pull/11942/files/58bf7617..7c44121b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11942&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11942&range=00-01

  Stats: 9 lines in 1 file changed: 1 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11942.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11942/head:pull/11942

PR: https://git.openjdk.org/jdk/pull/11942