compiler: Prevent undefined Temp through global init

[mloubout]

See test, basically prevents

    if ((time)%(ctf) == 0)
    {
      const float r0 = cos(time);
    ...
    }
   if (some conditon) {
    ...
    }
   if ((time)%(ctf) == 0){
     u[x] = r0
    }  

and instead in the presence of guards for scalar temps do

    float r0 = 0;
    if ((time)%(ctf) == 0)
    {
      r0 = cos(time);
    ...
    }
   if (some conditon) {
    ...
    }
   if ((time)%(ctf) == 0){
     u[x] = r0
    }  

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Codecov Report

❌ Patch coverage is 24.67532% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.14%. Comparing base (8ad5ec1) to head (5581b6d).

Files with missing lines	Patch %	Lines
tests/test_dse.py	8.62%	53 Missing ⚠️
devito/passes/clusters/aliases.py	69.23%	2 Missing and 2 partials ⚠️
devito/types/basic.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (8ad5ec1) and HEAD (5581b6d). Click for more details.

HEAD has 8 uploads less than BASE

Flag	BASE (8ad5ec1)	HEAD (5581b6d)
	18	10

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2858       +/-   ##
===========================================
- Coverage   78.96%   45.14%   -33.83%     
===========================================
  Files         248      248               
  Lines       51089    51158       +69     
  Branches     4422     4425        +3     
===========================================
- Hits        40344    23093    -17251     
- Misses       9937    27130    +17193     
- Partials      808      935      +127     

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