[llvm] [RISCV] Set riscv-fpimm-cost threshold to 3 by default (PR #159352)

[Alex Bradbury via llvm-commits]

asb wrote:

For completeness, the dynamic instcount impact on SPEC (rva22u64) are as follows. So always less than 1%, up a little in some benchmarks down in others. In general you'd expect this to be a small increase given constant pool accesses are typically two instructions (but as noted elsewhere, the fact we have issues with constant pool accesses means that reducing the number of constant pool entries can be a win).

```
Benchmark                  Baseline         This PR   Diff (%)
============================================================
500.perlbench_r         180654011583    180667466323      0.01%
502.gcc_r               221278049969    221277715808     -0.00%
505.mcf_r               134656204048    134656203896     -0.00%
508.namd_r              215796365527    217646645213      0.86%
510.parest_r            291717117181    291731988951      0.01%
511.povray_r             30915292688     30982784188      0.22%
519.lbm_r                91960161997     91217999812     -0.81%
520.omnetpp_r           137702932367    137704011614      0.00%
523.xalancbmk_r         283902048614    284735347946      0.29%
525.x264_r              379108027978    379107521490     -0.00%
526.blender_r           658543304053    659391437397      0.13%
531.deepsjeng_r         350038121606    350038121655      0.00%
538.imagick_r           238524616514    238568674630      0.02%
541.leela_r             405661034688    405660852859     -0.00%
544.nab_r               397053976830    398215801713      0.29%
557.xz_r                129832192055    129832192046     -0.00%
```

https://github.com/llvm/llvm-project/pull/159352