RFR: 8315545: C1: x86 cmove can use short branches

[Aleksey Shipilev]

Spotted this minor inefficiency when looking at profiling code. C1 profiling code uses `cmove`-s often, and often with immediates, so `LIR_Assembler::cmove` ends up emitting the real jump over the alternative branch. But the alternative branch is just `{reg,stack,const}2reg` conversion, and AFAICS is guaranteed to be short. Therefore, we can use the short branch for that forward jump. 

Sample `cmov` disassembly:


# Before
   0.19%          0x00007f1c98cc2d1d:   cmp    $0x0,%eax
              ╭   0x00007f1c98cc2d20:   jne    0x00007f1c98cc2d2b
              │   0x00007f1c98cc2d26:   mov    $0x2,%ecx
              ↘   0x00007f1c98cc2d2b:   and    $0x3ffe,%ecx
   0.54%          0x00007f1c98cc2d31:   cmp    $0x0,%ecx

# After
   0.17%          0x00007fcf18dda199:   cmp    $0x0,%eax
              ╭   0x00007fcf18dda19c:   jne    0x00007fcf18dda1a3
              │   0x00007fcf18dda19e:   mov    $0x2,%ecx
              ↘   0x00007fcf18dda1a3:   and    $0x3ffe,%ecx
   1.24%          0x00007fcf18dda1a9:   cmp    $0x0,%ecx


There are some code space savings, visible even on trivial `-Xcomp -XX:TieredStopAtLevel=... HelloWorld`.


# Before
tier1: nmethod total size        :   430104 bytes
tier2: nmethod total size        :   467336 bytes
tier3: nmethod total size        :   923368 bytes

# After
tier1: nmethod total size        :   429000 bytes (-0.25%)
tier2: nmethod total size        :   466016 bytes (-0.28%)
tier3: nmethod total size        :   915632 bytes (-0.84%)


Additional testing:
 - [ ] Linux x86_64 fastdebug `tier1 tier2 tier3`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/15537/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15537&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315545
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15537.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15537/head:pull/15537

PR: https://git.openjdk.org/jdk/pull/15537