nir: Don't assert on truncating tg4 offsets.
From the GL 4.6 spec: "If the value of any non-ignored component of the offset vector operand is outside implementation-dependent limits, the results of the texture lookup are undefined." We shouldn't assertion fail, then.
Fixes a crash in a piglit test that supplies a negative random number up to 10,000 as the first coordinate for some reason (@llandwerlin do you remember why?).