Does your piglit have the following commit?  Mesa was updated to follow the spec, and the test was previously incorrect.

commit 63aef6af996d5ca37e9dee78e0392106848e2735
Author: Neil Roberts <neil@linux.intel.com>
Date:   Wed Nov 26 18:35:32 2014 +0000

    Don't test floats with the form ‘1f’
    
    There is nothing in the GLSL spec which says you can have a float
    literal without an exponent or a decimal point even if you add the ‘f’
    suffix. This patch replaces that literal with some other variations
    that should be accepted according to the spec.
    
    Reviewed-by: Matt Turner <mattst88 at gmail.com>

This is the version of Piglit that is being used:
http://cgit.freedesktop.org/piglit/commit/?id=1f39e07d44f7d78557d69c505168afb8eab913c6

Here is the failing test output:
06/13 08:50:32.209 DEBUG|        base_utils:0239| [stdout] + Running test [shaders/glsl-floating-constant-120] of expected runtime 0.0 sec: [bin/shader_runner tests/shaders/glsl-floating-constant-120.shader_test -auto]
06/13 08:50:32.310 DEBUG|        base_utils:0239| [stderr] Failed to compile FS: 0:5(14): error: syntax error, unexpected NEW_IDENTIFIER, expecting ',' or ';'
06/13 08:50:32.312 DEBUG|        base_utils:0239| [stderr] 
06/13 08:50:32.314 DEBUG|        base_utils:0239| [stdout] PIGLIT: {"result": "fail" }
06/13 08:50:32.315 DEBUG|        base_utils:0239| [stdout] + fail :: shaders/glsl-floating-constant-120

Brian Wilson said:
> It fails in the following Mesa commit:
>   c2a0600 i965: Don't set NirOptions for stages that will use the vec4 backend [May 2015-ish]

Incorrect. The actual Mesa tree that Brian's team is validating is commit c1a0600 plus many patches. I've the pushed the full tree, patches included, to 

  git://github.com/chadversary/mesa refs/tags/chadv/cros-gerrit-262788-patched

Also, the Piglit being tested is the following commit plus a few incosequential patches.

  commit 4069bec62a8a99c44573395ea1597694eeee760f
  Author:     Marek Olšák <marek.olsak@amd.com>
  AuthorDate: Thu Apr 30 14:11:26 2015 +0200
  Commit:     Marek Olšák <marek.olsak@amd.com>
  CommitDate: Thu Apr 30 22:41:02 2015 +0200
  Subject: framework: use the correct executable for GLES parser tests

Building that Piglit against recent Mesa, however, produces an error at linktime. The fix requires backporting Piglit commit 845ba0e4dfb9372307f5d3032abdd0860e76731b.

I was able to reproduce this bug using the old versions of piglit and mesa

  piglit:  1f39e07d44f7d78557d69c505168afb8eab913c6
  mesa:    c2a0600

After updating piglit to the latest revision, the test passes.  This
seems to be the correct behavior, based on Ian's comment.

Based on this information, I think it is safe to conclude that this
bug does not indicate a regression in mesa.  Instead, it is caused by
using a fixed version of mesa, but failing to pull in the
corresponding piglit fix.

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