float printing was lossy in 2 places, fix this

[regehr]

ok let's try this again. the problem was in to_string(fp) -- this is lossy, it does not faithfully print the value. this patch should result in fully round-trippable float printing.

there was an analogous problem in CEX printing, this fixes that too

[regehr]

here's an example that justifies both parts of this patch:

define float @test_float_fdiv_pzero_f32_out() #0 {
  %result = fdiv float 0x3810000000000000, -2.000000e+00
  ret float %result
}

attributes #0 = { denormal_fpenv(positivezero|ieee) }

current alive2:

----------------------------------------
define float @test_float_fdiv_pzero_f32_out() {
#0:
  %result = fdiv float 0.000000, -2.000000
  ret float %result
}
=>
define float @test_float_fdiv_pzero_f32_out() nofree noundef willreturn memory(none) {
#0:
  ret float 0.000000
}
Transformation doesn't verify!

ERROR: Value mismatch

NOTE: The counterexample is unique.

Example:

Source:
float %result = #x80400000 (-0.000000000000?)

Target:
Source value: #x80400000 (-0.000000000000?)
Target value: #x00000000 (+0.0)

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors

alive2 with this patch:

----------------------------------------
define float @test_float_fdiv_pzero_f32_out() {
#0:
  %result = fdiv float 1.1754944e-38, -2e+00
  ret float %result
}
=>
define float @test_float_fdiv_pzero_f32_out() nofree noundef willreturn memory(none) {
#0:
  ret float 0e+00
}
Transformation doesn't verify!

ERROR: Value mismatch

NOTE: The counterexample is unique.

Example:

Source:
float %result = #x80400000 (-5.877472e-39)

Target:
Source value: #x80400000 (-5.877472e-39)
Target value: #x00000000 (+0.0)

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors

[regehr]

btw I think it's not LLVM who's wrong on this example, it's us