When constant double 0.0 is lowered, make sure 0 is copied directly from an

When constant double 0.0 is lowered, make sure 0 is copied directly from an
integer register to a floating point register. It is not valid to interpret
the value of a floating pointer register as part of a double precision
floating point value after a single precision floating point computational
or move instruction stores its result to the register.

- In the test case, the following code is generated before this patch is
  applied:
mtc1  $zero, $f2    ; unformatted copy to $f2
mov.s $f0, $f2      ; $f0 is in single format
sdc1  $f12, 0($sp)
mov.s $f1, $f2      ; $f1 is in single format
c.eq.d  $f12, $f0   ; $f0 cannot be interpreted as double

- The following code is generated after this patch is applied:
mtc1  $zero, $f0    ; unformatted copy to $f0
mtc1  $zero, $f1    ; unformatted copy to $f1
c.eq.d  $f12, $f0   ; $f0 can be interpreted as double

Bhanu Chetlapalli and Chris Dearman at MIPS technologies reported this bug and
provided the test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137484 91177308-0d34-0410-b5e6-96231b3b80d8